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Sci Transl Med-2011-Rozen-92ps31

B ETA DEFENSIN 126An estimated 10 to 15% of couples at some point su er from infertility, which is de ned as the inability to conceive a er 1 year of unprotected intercourse (1). O en, multiple factors contribute to infertility, and many treatment decisions rely on trial and error (2–4). In many couples, genetically deter-mined characteristics of the male’s semen probably play at least some role, and this may be compounded by reduced fertility in the female partner. However, little is known about the genetic determinants of male fer-tility beyond e ects mediated by reduced sperm count (5–9), as well as a few addition-al rare variants that account for a small frac-tion of male infertility (10–12). In this issue of Science Translational Medicine , Tollner et al . (13) o er a welcome advance showing that a common variant in the gene encoding beta defensin 126, DEFB126, probably con-tributes to infertility.DEFB126 is a member of a diverse yet underinvestigated family of genes, many of which encode antimicrobial proteins pres-ent in the skin or mucous membranes (14). Some beta defensins are expressed primarily in the epididymis (15), a small organ on the side of each testis that serves to store sperm and contributes to their maturation (Fig. 1A). It is hypothesized that some of the beta defensins expressed in the epididymis help to protect the organ from infection-causing microbes. However, among the epididymal beta defensins, the DEFB126 protein has a di erent function. In a model organism, itcoats the plasma membrane of sperm (16)(Fig. 1B). Functional studies in macaques show that the DEFB126 protein coat helps sperm to penetrate cervical mucus (17), shields them from recognition by the female immune system (18), and promotes their at-tachment to the epithelium of the oviduct (fallopian tube) (Fig. 1A) (19).A COMMON FUNCTIONAL GENETICVARIANT IN DEFB126In examining the nucleotide sequence of hu-man DEFB126, Tollner and colleagues no-ticed an extremely common sequence variant (13). is variant, denoted “del,” involves the deletion of two cytosine nucleotides starting at position 317 of the reference coding se-quence (CDS) of the gene (Concensus CDS no. 12990.1). us, although the reference variant encodes a predicted 111–amino acid protein, the del variant results in a frame shi that removes the stop codon. is results in a “nonstop” mRNA in which the last six amino acids of the reference protein product (PVSPTG) are predicted to be replaced by a 26–amino acid sequence: RFSHWLNIP AS-VSCSRIP DSLKQRGL. e authors further reported that in the epididymis, a del/del homozygote had DEFB126 mRNA levels that were only 10% of those in a reference-allele homozygote (13). ey hypothesize that this might be the result of suppression of the non-stop mRNA by cellular surveillance mecha-nisms. In any case, the surface of sperm from del/del homozygotes showed dramati-cally less glycosylation of the type associated with DEFB126 (20)—speci cally, O-linked galactose-GalNAc (N -acetylgalactosamine) glycans (13). is lack of sperm glycosylation is presumably due to reduced levels of the DEFB126 protein itself, although this was notmeasured directly. Importantly, the amount of this coating correlates with the ability of sperm to penetrate hyaluronic acid (HA) (13). Because HA is an established surrogate for cervical mucus (21, 22), the near inability of sperm from del/del men to penetrate HA indicates that these sperm would have great di culty penetrating cervical mucus.DEL/DEL AND FERTILITYNaturally, some questions then followed: Does homozygosity for the del variant im-pede sperm penetration of cervical mucus or other functions mediated by DEFB126 in vivo? Furthermore, does this then a ect fer-tility? To address the latter question, Tollner et al . studied DEFB126 genotypes and fertil-ity in approximately 500 newlywed couples (13). ey found that a er 2 years, couples in which the man’s genotype was del/del were signi cantly less likely to have had a pregnancy than were the other couples. A second, con rmatory analysis found that in any given month, couples in which the man’s genotype was del/del were 30% less likely than were the other couples to have a birth. e con uence of the experimental and human genetic studies provides strong evidence that men with the del/del genotype have reduced fertility. Nevertheless, the ge-netic ndings will need to be replicated in a larger cohort of couples or over a longer period of time before we can be con dent in the e ects of the del/del DEFB126 genotype on male fertility.It is also important to recognize that re-duced penetration of cervical mucus might not be the only reason that del/del homo-zygotes have lower fertility. In macaques, DEFB126 also helps sperm avoid recogni-tion by the female immune system (18). In addition, DEFB126 promotes sperm attach-ment to the epithelium of the oviduct (19), the organ in which fertilization normally takes place. us, beyond reducing sperm penetration of cervical mucus, the del/del genotype could conceivably contribute to infertility by abrogating these or other, still-unknown, functions of DEFB126. e geographically widespread, high prevalence of the del variant—close to 50% of alleles in populations from Asia, Europe, and Africa—is surprising in light of the ob-served reduced fertility of del/del homozy-gotes. Fundamentally, we can only specu-late about why the del variant is extremely common in so many populations. Tollner and colleagues show possible evidence ofI N F E R T I L I T YDefending Male FertilitySteve Rozen**E-mail: steve.rozen@.sgDuke–NUS Graduate Medical School Singapore, 8 Col-lege Road, 169857 Singapore. A n estimated 10 to 15% of couples su er from infertility, and many treatment decisions rely on trial and error. In this issue of Science Translational Medicine , Tollner and colleagues provide strong evidence from a human genetics study that a common variant in the beta defensin 126 gene, the “del” variant, can reduce male fertility substantially. In addition, they show a plausible mechanism for reduced fertility: Sperm from del/del homozygotes lack an important component of their glycoprotein coat and have di culty penetrating a surrogate for cervical mucus. If replicated in future studies, these ndings promise to guide choices about the timing and type of assisted reproduction interventions—and further hint at the possibility of treating sperm from del/del homozygotes to promote fertility.o n J u l y 21, 2011s t m .s c i e n c e m a g .o r g D o w n l o a d e d f r o mC R ED I T : P . H UE Y /S C I E N C E T R A N S L A T I O N A L M E D I C I N Ebalancing selection, in which heterozygotes mig ht have a selective advantag e over ho-mozygotes for either allele. But the authors’ genetic studies (13) show no evidence that heterozy otes have any advanta e over reference-allele homozyg otes in terms of time to pregnancy or birth. us, any het-erozygote advantage would have to be medi-ated by other e ects, such as, speculatively, resistance to infection.CLINICAL IMPLICATIONS e g enetic nding s by Tollner et al . o er two clinical opportunities. A future oppor-tunity is the possibility of treating sperm from del/del men to restore g lycosylated DEFB126. In previous work, the authors showed that it is possible to remove and then restore the DEFB126 coat from ma-caque sperm (Fig. 1B) (23). Restoring g ly-cosylated DEFB126 would depend on iden-tifying an abundant source of the protein with the necessary pattern of glycosylation. e more immediate clinical opportunity is that of guiding choices about the timing andtype of assisted reproduction intervention (Fig. 1A). In g eneral terms, if it is known that time to preg nancy is likely to be longbecause the man has the del/del genotype, then resorting immediately to assisted re-production may be attractive. is would beespecially true if the woman has dwindlingovarian reserves and is also contributing to the couple’s infertility. is situation is be-coming more common as women increas-ing ly delay childbearing , which results indecreased female fertility (24–28).If it is con rmed that the del/del geno-type depresses fertility by impeding sperm penetration of the cervical mucus, then apossible intervention is intrauterine insemi-nation (Fig. 1A). In this technique, washed sperm are deposited directly into the uterusvia a catheter inserted throug h the cervi-cal canal, thus mechanically bypassing the cervical mucus. is can be combined withovarian hyperstimulation to increase thechances of successful fertilization. If, how-ever, it turns out that the del/del genotypedepresses fertility through additional mech-anisms [such as poor evasion of the femaleimmune system (18) or suboptimal interac-tions with the oviductal epithelium (19)],possible interventions would include in vi-tro fertilization (IVF) or intracytoplasmicsperm injection (ICSI) (Fig. 1A). For IVF,the sperm and an egg are combined in a mi-crodroplet, and the sperm can fertilize the egg naturally, albeit outside of the body. For ICSI, the sperm is injected directly into the cytoplasm of the egg. For both assisted re-production techniques, embryos are placed in the uterus a few days a er fertilization; if an embryo implants, a pregnancy results.With either technique, any requirement for the DEFB126 protein coat would be by-passed because DEFB126 must be removedfrom the sperm as a nal step before fertil-ization (23).It is too soon to predict how this new knowledg e about the DEFB126 g enotype(13) may aect the multifaceted decisions reg arding the use of assisted reproduc-tion technolog ies for human preg nancy. Itis clear that this genetic information could lead to more informed assisted reproduc-tion, in addition to possible new methods of treating sperm. In the meantime, we eagerly await a more complete understanding of the epidemiolog y of the DEFB126 del variant and of the mechanisms by which it increases time to pregnancy.REFERENCES AND NOTES 1. M. G. Hull, C. M. Glazener, N. J. Kelly, D. I. Conway, P. A.Foster, R. A. Hinton, C. Coulson, P. A. Lambert, E. M. Watt,K. M. Desai, Population study of causes, treatment, andoutcome of infertility. Br. Med. J. (Clin. Res. Ed.) 291, 1693–1697 (1985).2. P. J. Rowe, F. H. Comhaire, T. B. Hargreave, H. J. Mellows, WHO Manual for the Standardized Investigation and Diag-nosis of the Infertile Couple (Cambridge Univ. Press, Cam-birdge, 1993).3. American Urological Association Infertility Best P ractice Statement P anel, ed., 2010. “The Optimal Evaluation of the Infertile Male: AUA Best P ractice Statement,” down-Fig. 1. DEFB126 in the human male and female reproductive tracts. (A ) Sperm are created in thetestis and subsequently stored in the epididymis. Here, they also obtain a coating of glycoproteins,of which DEFB126 is a major component. After ejaculation, the sperm ﬁ nd themselves at the open-ing of the cervical canal, which is ﬁ lled with mucus. The sperm must swim through the cervical mucus and make their way through the uterus to the oviducts (fallopian tubes), where they mightmanage to fertilize an egg. DEFB126 helps the sperm to penetrate the cervical mucus, probably helps them to evade the female immune system, and promotes their attachment to the epithelial lining of the oviducts. (B ) Macaque sperm stained with an antibody for DEFB126. Reproduced with permission from (16). CervixFertile sperm with DEFB126 coating Less-fertile sperm, which lack the DEFB126 coatingClinical implication: Treat sperm to provide a coating of glycosylated DEFB126Location of cervical mucusUterusOvary Oviduct AB Oviduct OvaryTestisEpididymisClinical implication: Intrauterine insemination Clinical implications:In vitro fertilizationIntracytoplasmic sperm injectiono n J u l y 21, 2011s t m .s c i e n c e m a g .o r g D o w n l o a d e d f r o mloaded from /content/media/op-timalevaluation2010.pdf.4. P. J. Rowe, F. H. Comhaire, T. B. Hargreave, A. M. A. Mah-moud, WHO Manual for th e Standardized Investigation and Diagnosis of the Infertile Male (Cambridge Univ. Press, Cambridge, 2000).5. C. Krausz, S. Degl’Innocenti, Y chromosome and male in-fertility: Update, 2006. Front. Biosci.11,3049–3061(2006).6. M. J. Noordam, S. Repping, The human Y chromosome:A masculine chromosome. Curr. Opin. Genet. Dev.16,225–232 (2006).7. M. Simoni, E. Bakker, C. Krausz, EAA/EMQN best practiceguidelines for molecular diagnosis of y-chromosomal microdeletions. State of the art 2004. Int. J. Androl.27, 240–249 (2004).8. S. Repping, H. Skaletsky, J. Lange, S. Silber, F. Van Der Veen,R. D. Oates, D. C. Page, S. Rozen, Recombination between palindromes P5 and P1 on the human Y chromosome causes massive deletions and spermatogenic failure. Am.J. Hum. Genet.71, 906–922 (2002).9. T. Kuroda-Kawaguchi, H. Skaletsky, L. G. Brown, P. J. Minx,H. S. Cordum, R. H. Waterston, R. K. Wilson, S. Silber, R.Oates, S. Rozen, D. C. Page, The AZFc region of the Y chro-mosome features massive palindromes and uniform re-current deletions in infertile men. Nat. Genet.29, 279–286 (2001).10. A. Anguiano, R. D. Oates, J. A. Amos, M. Dean, B. Gerrard, C.Stewart, T. A. Maher, M. B. White, A. Milunsky, Congenital bilateral absence of the vas deferens. A primarily genital form of cystic ﬁ brosis. JAMA267, 1794–1797 (1992). 11. R. J. Aitken, A. Ross, M. M. Lees, Analysis of sperm functionin Kartagener’s syndrome. Fertil. Steril.40, 696–698 (1983).12. B. A. Afzelius, R. Eliasson, O. Johnsen, C. Lindholmer, Lackof dynein arms in immotile human spermatozoa. J. Cell Biol.66, 225–232 (1975).13. T. L. Tollner, S. A. Venners, E. J. Hollox, A. I. Yudin, X. Liu, G.Tang, H. Xing, R. J. Kays, T. Lau, J. W. Overstreet, X. Xu, C. L.Bevins, G. N. Cherr, A common mutation in the defensinDEFB126 causes impaired sperm function and subfertility.Sci. Transl. Med.3, 92ra65 (2011).14. R. I. Lehrer, Primate defensins. Nat. Rev. Microbiol.2, 727–738 (2004).15. Y. Yamaguchi, T. Nagase, R. Makita, S. Fukuhara, T. Tomita,T. Tominaga, H. Kurihara, Y. Ouchi, Identiﬁ cation of mul-tiple novel epididymis-speciﬁ c beta-defensin isoforms inhumans and mice. J. Immunol.169, 2516–2523 (2002).16. A. I. Yudin, T. L. Tollner, M. W. Li, C. A. Treece, J. W. Over-street, G. N. Cherr, ESP13.2, a member of the beta-defen-sin family, is a macaque sperm surface-coating proteininvolved in the capacitation process. Biol. Reprod.69,1118–1128 (2003).17. T. L. Tollner, A. I. Yudin, C. A. Treece, J. W. Overstreet, G. N.Cherr, M acaque sperm coating protein DEFB126 facili-tates sperm penetration of cervical mucus. Hum. Reprod.23, 2523–2534 (2008).18. A. I. Yudin, S. E. Generao, T. L. Tollner, C. A. Treece, J. W.Overstreet, G. N. Cherr, Beta-defensin 126 on the cell sur-face protects sperm from immunorecognition and bind-ing of anti-sperm antibodies. Biol. Reprod.73, 1243–1252(2005).19. T. L. Tollner, A. I. Yudin, A. F. Tarantal, C. A. Treece, J. W.Overstreet, G. N. Cherr, Beta-defensin 126 on the surfaceof macaque sperm mediates attachment of sperm to ovi-ductal epithelia. Biol. Reprod.78, 400–412 (2008).20. A. I. Yudin, C. A. Treece, T. L. Tollner, J. W. Overstreet, G. N.Cherr, The carbohydrate structure of DEFB126, the majorcomponent of the cynomolgus Macaque sperm plasmamembrane glycocalyx. J. Membr. Biol.207, 119–129(2005).21. R. J. Aitken, H. Bowie, D. Buckingham, D. Harkiss, D. W.Richardson, K. M. West, Sperm penetration into a hyal-uronic acid polymer as a means of monitoring functionalcompetence. J. Androl.13, 44–54 (1992).22. S. Tang, C. Garrett, H. W. Baker, Comparison of human cer-vical mucus and artiﬁ cial sperm penetration media. Hum.Reprod.14, 2812–2817 (1999).23. T. L. Tollner, A. I. Yudin, C. A. Treece, J. W. Overstreet, G. N.Cherr, Macaque sperm release ESP13.2 and PSP94 duringcapacitation: The absence of ESP13.2 is linked to sperm-zona recognition and binding. Mol. Reprod. Dev.69, 325–337 (2004).24. S. Sunderam, J. Chang, L. Flowers, A. Kulkarni, G. Sentelle,G. Jeng, M. M acaluso; Centers for Disease Control andPrevention (CDC), Assisted reproductive technology sur-veillance—United States, 2006. MMWR Surveill. Summ.58,1–25 (2009).25. A. Nyboe Andersen, V. Goossens, A. P. Ferraretti, S. Bhat-tacharya, R. Felberbaum, J. de M ouzon, K. G. Nygren;The European IVF-monitoring (EIM) Consortium, forthe European Society of Human Reproduction and Em-bryology (ESHRE), Assisted reproductive technology inEurope, 2004: Results generated from European registersby ESHRE. Hum. Reprod.23, 756–771 (2008).26. S. C. Tough, C. Newburn-Cook, D. W. Johnston, L. W. Sven-son, S. Rose, J. Belik, Delayed childbearing and its impacton population rate changes in lower birth weight, mul-tiple birth, and preterm delivery. Pediatrics109, 399–403(2002).27. H.-P. Kohler, F. C. Billari, J. A. Ortega, The emergence oflowest-low fertility in Europe during the 1990s. Popul. Dev.Rev.28, 641–680 (2002).28. P. Katz, R. Nachtigall, J. Showstack, The economic impactof the assisted reproductive technologies. Nat. Cell Biol.4,(Suppl. 1), S29–S32 (2002).29. Funding: The author is supported by the Singapore Min-istry of Health and the Agency for Science, Technology,and Research. Competing Interests: The author declaresno competing interests.Citation: S. Rozen, Defending male fertility. Sci. Transl. Med.3, 92ps31 (2011).10.1126/scitranslmed.3002743onJuly21,211stm.sciencemag.orgDownloadedfrom。

igcse英语article范文

igcse英语article范文Title: The Impact of Social Media on TeenagersIn today's digital age, social media has become an integral part of most teenagers' lives. With the rise of platforms such as Instagram, Snapchat, and TikTok, young people are constantly connected and exposed to a world of information and communication. While social media has its undeniable benefits, it also poses various challenges and risks, particularly for teenagers.One of the biggest impacts of social media on teenagers is its influence on mental health. Studies have shown that excessive use of social media can lead to feelings of loneliness, anxiety, and depression among young people. The constant need for validation through likes and comments can also trigger feelings of insecurity and inferiority in teenagers, as they compare themselves to others on social media.Furthermore, social media has also been found to contribute to cyberbullying, which can have devastating effects on teenagers' mental and emotional well-being. With the anonymity that social media provides, bullies can easily targettheir victims without fear of consequences, leading to a rise in cases of online harassment and abuse.Another impact of social media on teenagers is its effect on interpersonal relationships. While social media allows teenagers to stay connected with friends and family members, it can also hinder face-to-face communication and intimacy. Many young people are more comfortable expressing themselves through texts and emojis than in person, leading to a decline in social skills and emotional intelligence.In addition, social media can also have a negative impact on teenagers' academic performance. With the constant distraction of notifications and updates, many young people find it difficult to focus on their studies and assignments. The pressure to maintain a social media presence and keep up with the latest trends can also consume a significant amount of teenagers' time and energy, leaving little room for academic pursuits.Despite these challenges, social media also has its benefits for teenagers. It can provide a platform for self-expression, creativity, and activism, allowing young people to share their ideas and opinions with a global audience. Social media has also been instrumental in raising awareness about social issues,promoting diversity and inclusion, and mobilizing youth-led movements for change.In conclusion, the impact of social media on teenagers is complex and multifaceted. While it offers numerous opportunities for connection and communication, it also poses risks and challenges for young people's mental health, relationships, and academic performance. As such, it is essential for parents, educators, and policymakers to be mindful of the effects of social media on teenagers and work together to promote healthy and responsible use of these platforms.References:- Twenge, J.M. & Campbell, W.K. (2018). Associations between screen time and lower psychological well-being among children and adolescents: Evidence from a population-based study. Preventive Medicine Reports, 12, 271-283.- Kowalski, R.M., Giumetti, G.W., Schroeder, A.N., & Lattanner, M.R. (2014). Bullying in the digital age: A critical review and meta-analysis of cyberbullying research among youth. Psychological Bulletin, 140(4), 1073-1137.。

冠词 Article 的用法

in hospital in the hospital
在住院在医院里
in school in the school
在学习；在学校上课在学校里
out of question 不成问题；豪无疑问 out of the question 不可能；办不到
go to school
上学
go to the school 到学校去
6）在称呼或表示官衔，职位的名词前不加冠词： The guards took the American to General Lee. 士兵们把这个美国人送到李将军那里。
7）在三餐、球类运动和娱乐运动的名称前，不加冠词如：
have breakfast，play chess 8）当两个或两个以上名词并用时，常省去冠词：
go to hospital go to the hospital
go to bed go to the bed
去看病到医院去
去睡觉向那张床走去
5
冠词用法歌诀
名词是秃子，常要戴帽子。可数名词单，须用a或an。辅音前用a，an用元音前。若为特指时，则须用定冠。复数不可数，泛指the不见。碰到代词时，冠词均不现。
12）如果名词前已有this, that, these, those, whose, which, my, your, their, his, her, it词，意义不同
试比较：
at table at the table
在进餐在餐桌旁
Where do you live? I live on the second floor. 你住在哪？我住在二层。
That‘s the very thing I’ve been looking for. 那正是我要找的东西。

WEARING ARTICLE

专利名称：WEARING ARTICLE发明人：ICHIKAWA, Makoto,OHASHI, Naoto 申请号：JP2008066675申请日：20080916公开号：WO09/054206P1公开日：20090430专利内容由知识产权出版社提供摘要：A wearing article in which a front waist region and a rear waist region can be easily and accurately engaged with each other. A first fastening element (13) is installed on a front edge section (11) of the front waist region (8), and a second fastening element (14) engageable with the first fastening element (13) is installed on a rear edge section (12) of the rear waist region (9). The second fastening element (14) has a bellows structure extending in the lateral direction (X) and having crests (17) spaced in the longitudinal direction (Y). Even if a bellows region (16) where the bellows structure is formed makes contact with the first fastening element (13) with the bellows contracted, they are engaged with each other with a weak force and can be easily disengaged from each other. When the bellows region (16) is stretched in the longitudinal direction (Y) and made to make contact with the first fastening element (13), they can be in contact with each other in a large area to be firmly engaged with each other.申请人：ICHIKAWA, Makoto,OHASHI, Naoto地址：JP,JP,JP国籍：JP,JP,JP代理机构：SHIRAHAMA, Yoshiharu更多信息请下载全文后查看。

山东大学博士学位论文怀特的历史书...

山东大学博士学位论文海登?怀特的历史书写理论与文学观念姓名:杨杰申请学位级别:博士专业:文艺学指导教师:马龙潜20060320山东大学博士学位论文中文提要海登?怀特是当代美国著名思想史家、历史学家、文学批评家,他主导了世纪年代以后的历史哲学领域中的语言学转向,并将历史意识与历史方法引入文学批评领域,成为跨学科研究的典范。

怀特享誉世界,是新历史主义文学批评研究所不能忽视的重要理论家,称其为“理论家”的原因是??在新历史主义“庞杂”的队伍中只有他对“历史”进行了系统化的理论阐述,而其他人如格林布拉特等更擅长批评实践而非理论阐述。

怀特的理论在史学界激起轩然大波,褒贬不一。

可是,失之东隅,收之桑榆,其理论竟成了文学批评界的必读经典。

怀特理论虽被广泛引用,但对其学术进行较为全面、深入研究的却是相当有限,我们能查阅到的仅仅是在对新历史主义文学批评进行宏观研究时所做的篇幅不过是一节的介绍,至于更为细致、深入的针对其个人的研究却是相当稀少。

鉴于此,不论是否赞赏怀特的理论观点,都应回到对其理论本身的探讨上来,在对其理论进行较为全面分析、鉴别的基础之上再作出得失评判。

这不失为一种科学的态度。

选取怀特的历史书写理论为研究课题不是一件轻松的事情。

因为怀特知识渊博,学术功底深厚,不仅涉及多个学科领域知识,而且还大量论及学界泰斗的理论观点,其中的如海德格尔、伽达默尔、杰姆逊、福柯、利科等人的理论本身就很深奥,再加上怀特本人的生涩文笔,更令人有仰之弥高之感;尤其是在查阅他的原文资料时经常遇到夹杂意大利文、法文、德文等非英文词句,这也增加了理解的难度。

怀特的理论代表了“回归历史”的呼声。

历史问题作为人类本体存在的时间维度必然引发我们的思考,更是文学研究不能回避、也绝对无法回避的问题。

他的理论具有“多声部?复调”①式的特点,可以概括为“一个核心、两个维度、两种视角”。

所谓的“一个核心”是指怀特始终坚持文史相通的观念;“两个维度”指形式主义与“回归历史”是支撑其理论大厦的两个轴;“两种视角”是指“在文学中审视历史”和“在历史中研究文学”。

(完整版)CISG中英文对照版

联合国国际货物销售合同公约中英文对照字体大小：大- 中- 小quanzhoufanyi 发表于08-08-07 10:42 阅读(288) 评论(0) 联合国国际货物销售合同公约(United Nations Convention on Contractsfor the International Sale of Goods (1980) )PreambleThe States Parties to this Convention Bearing in Mind the broad objectives in the resolutions adopted by the sixth special session of the General Assembly of the United Nations on the establishment of a New International Economic Order. Considering that the development of international trade on the basis of equality and mutual benefit is an important element in promoting friendly relations among States, Being of the Opinion that the adoption of uniform rules which govern contracts for the international sale of goods and take into account the different social, economic and legal systems would contribute to the removal of legal barriers in international trade and promote the development of international trade, have decreed as follows: 本公约个缔约国: 铭记联合国大会第六界特别会议通过的关于建立新的国际经济次序的各项决议的广泛目标, 考虑到在平等互利基础上发展国际贸易, 是促进各国间友好关系的一个重要因素, 认为采用照顾到不同的社会, 经济和法律制度的国际货物销售合同统一规则，将有助于减少国际贸易的法律障碍, 促进国际贸易的发展. 兹协议如下.PART ISphere of Application and General ProvisionsChapter ISphere of ApplicationArticle 1(1) This Convention applies to contracts of sale of goods between parties whose places of business are in different States: ( 本公约适用于营业地在不同国家的当事人之间所订立的货物销售合同，)(a) when the States are Contracting States; or ( 如果这些国家是缔约国, 或)(b) when the rules of private international law lead to the application of the law of a Contracting State. ( 如果国际私法规则导致适用某一缔约国的法律，)(2) The fact that the parties have their places of business in different States is to be disregarded whenever this fact does not appear either from the contract or from any dealings between, or from information disclosed by, the parties at any time before or at the conclusion of the contract. ( 当事人营业地在不同国家的事实，如果从合同或从订立合同前任何时候或订立合同时, 当事人之间的任何交易或当事人透露的情报均看不出, 应不予考虑)(3) Neither the nationality of the parties nor the civil or commercial character of the parties or of the contract is to be taken into consideration in determining the application of this Convention. ( 在确定本公约的适用时, 当事人的国籍和当事人或合同的民事或商业性质，应不予考虑) Article 2This Convention does not apply to sales: ( 本公约不适用以下的销售)(a) of goods bought for personal, family or household use, unless the seller, at any time before or at the conclusion of the contract, neither knew nor ought to have known that the goods were bought for any such use; ( 购供私人, 家人或家庭使用的货物销售, 除非卖方再订立合同前任何时候或订立合同时不知道而且没有理由知道这些货物是购供任何这种使用)(b) by auction; ( 经由拍卖销售的)(c) on execution or otherwise by authority of law; ( 根据法律执行令状或其他领状的销售)(d) of stocks, shares, investment securities, negotiable （可通过谈判解决的）instruments （手段）or money;( 公债，股票，投资证券, 流通票据或是货币的销售)(e) of ships, vessels （船只）, hovercraft （水翼船）or aircraft;( 船舶船只，气垫船或是飞机的销售)(f) of electricity. ( 电力的销售)Article 3(1) Contracts for the supply of goods to be manufactured or produced are to be considered sales unless the party who orders the goods undertakes to supply a substantial part of the materials necessary for such manufacture or production.( 供应尚待制造或生产的货物的合同应视为销售合同, 除非订购货物的当事人保证供应这种制造或生产所需的大部分重要材料.)(2) This Convention does not apply to contracts in which the preponderant( 优势的) part of the obligations of the party （当事人）who furnishes the goods consists in the supply of labour or other services.( 本公约不适用于供应货物一方的绝大部分义务在于供应劳力或其它服务的合同)Article 4This Convention governs only the formation （形成）of the contract of sale and the rights and obligations of the seller and the buyer arising from （产生）such a contract. In particular, except as otherwise expressly （明确地）provided in this Convention, it is not concerned with: ( 本公约只适用于销售合同的订立和卖方和买方因此种合同而产生的权利和义务. 特别是本公约除非另有明文规定, 与以下事项无关:)(a) the validity （效力）of the contract or of any of its provisions （供应）or of any usage;( 合同的效力，或其任何条款的效力，或任何惯例的效力)(b) the effect which the contract may have on the property in the goods sold. （合同对所销售物所有权可能产生的影响。

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Effect of shearing on crystallization behavior ofpoly(ethylene naphthalate)W.J.Yoon,H.S.Myung,B.C.Kim,S.S.Im *Department of Textile Engineering,Hanyang University,Haengdang,Seongdong,Seoul 133-791,South KoreaReceived 11August 1999;received in revised form 24September 1999;accepted 30September 1999AbstractThe effect of shear history on the isothermal crystallization behavior of poly(ethylene naphthalate)(PEN)was investigated by rheological and morphological measurements.Time sweep measurements of storage modulus (G H )and dynamic viscosity (h H )were carried out on the molten PEN by Advanced Rheometric Expansion System (ARES)in the parallel-plate geometry at several different temperatures and frequencies,followed by structural analysis by differential scanning calorimeter (DSC),X-ray diffractometer,and polarizing microscopy for the shear-induced crystallized PEN specimens in the ARES measurements.The rate of isothermal crystallization of PEN was notably affected by temperature,while the shear rate has an important effect on the structures of the resultant crystals.At a constant shear rate,the rate of crystallization by shear-induced structuring mechanism was increased with lowering temperature over the temperature range 230–250ЊC.The rate of crystallization was increased with increasing shear rate at a given temperature.An increase in shear rate increased both nucleation and number of crystallites.Further,it increased the content of the a -form crystal in the specimen.On the other hand,lower shear rate offered more favorable conditions for forming the b -form crystal.DSC analysis exhibited that the b -form crystal had higher melting temperature (T m )than the a -form crystal.The wide angle X-ray diffraction (WAXD)patterns also ascertained that higher content of the a -form crystal was produced in the PEN specimen crystallized at higher frequency.᭧2000Elsevier Science Ltd.All rights reserved.Keywords :Poly(ethylene naphthalate);Rheology;Shear-induced crystallization1.IntroductionShear-induced structural changes in polymeric materials take an increasing interest in the ﬁeld of polymer proces-sing.In real polymer processing very complex deformation histories are involved,which can inﬂuence ultimate proper-ties of plastics.Recent advances in experimental techniques that allow in situ measurements of materials under deforma-tion have escalated research in this subject area.It has been known for a long time that ﬂow stress have accelerating effect on the crystallization of semi-crystalline polymers [1–6].It is supposed that the application of a shear stress to a polymer melt should lead to formation of orientation and reduce the entropy of the melt,which results in a higher melting temperature and,hence,lead to an increased super-cooling [3,7].Several experiments have been described in the literature where attempts were made to quantify the shear stress-induced crystallization in molten semi-crystal-line polymers such as polypropylene [3,8,9],polyethylene oxide [10],polypropylene [11–13],and polybutene-1[3,14].Some investigators used rotational viscometers andmeasured either the volume change [15]or the number of nuclei formed during shearing [11,14].The polymers enum-erated above are apt to process because of low melting point and viscosity.On the other hand,PEN has good thermal and mechanical properties and is being used as engineering plastics.PEN is reported to have two different triclinic crystalline structures,a -form and b -form crystals.Of two crystal forms,the b -form crystal is known to be more stable than the a -form.The effect of crystallization temperature on the resultant crystal structure is well recognized;lower temperature favors formation of the a -form crystal.The critical temperature is reported about 230ЊC.However,the effect of shear history on the crystal structure of PEN has not been reported.In this study,the shear-induced crystallization behavior of PEN was investigated on the rheological basis.The effect of shear history on the crystalline structure was also discussed in terms of thermal and morphological properties.2.Experimental 2.1.MaterialThe PEN tested was a commercially available gradePolymer 41(2000)4933–49420032-3861/00/$-see front matter ᭧2000Elsevier Science Ltd.All rights reserved.PII:S0032-3861(99)00703-X*Corresponding author.Tel.:ϩ82-2-2292-0495;fax:ϩ82-2-2297-5859.E-mail address:imss007@email.hanyang.ac.kr (S.S.Im).supplied by Kolon Group in South Korea.The inherent viscosity,0.344dl/g was determined in a mixture of triﬂuoroacetic acid and chloroform (1/3v/v%)with an Ubbelohde viscometer at 25^0:1ЊC :The polymer was dried in a vacuum oven at 120ЊC for 24h prior to use.2.2.Measurement of physical propertiesThe dynamic rheological properties were measured by ARES (Rheometric Scientiﬁcs)in the parallel plate geome-try.The plate diameter was 12.5mm,strain level was 5%,and gap between the plates was 1mm.The PEN chips were melted at 300ЊC.The initial gap was set to a value equiva-lent to ﬁnal gap plus 50m m.The excess sample squeezed out by reducing the gap was carefully trimmed off.The value was reset to the ﬁnal gap value,1mm.To remove the residual stress the newly set PEN specimen was relaxed for about 5min at the temperature in nitrogen atmosphere,then cooled to the predetermined temperature for rheologi-cal measurements.A time-sweep experiment was continued for the specimen till the G H reached the ceiling value of the apparatus.After ARES measurement,the molten PEN sample was detached from the plates for measuring other properties such as thermal and morphological properties by DSC,X-ray diffractometer and polarizing optical micro-scopy.Thermal properties were measured by Perkin–Elmer DSC-7over the temperature 50–300ЊC at the heating rate of 10ЊC/min under nitrogen purge.The isothermalcrystallization experiment was performed by two different methods.Firstly,the PEN sample was heated to 300ЊC at the heating rate of 200ЊC/min,and held for about 5min,then they were cooled to the preset temperature to bring about the isothermal crystallization for same time required in ARES experiment.Secondly,the PEN chips were melted at 300ЊC between two slide glasses for 5min on the hot stage.They were moved to an oil bath very quickly and isothermally crystallized at 230,240,and 250ЊC for 4,10,and 24h,respectively.Wide angle X-ray diffraction patterns of the isothermally crystallized PEN specimen in the oil bath and ARES were obtained by X-ray diffractometer (Rigaku Denki)with Ni-ﬁltered CuK a radiation at 35kV and 35mA.Morphology of quiescent and shear-induced crystallized PEN specimen was observed by polarized microscopy (Nikon HFX-IIA).The spherulite structure was observed by microtoming the specimen.3.Results and discussionIn the plot of G H and h H versus time at a given frequency for a polymer,the two parameters may give information on the change in physicochemical properties of the polymer.For thermally sensitive polymer melts,an irreversible decrease of viscosity with time at a constant shear rate suggests the possibility of thermal degradation of polymer molecules,whereas an irreversible increase of viscosityW.J.Yoon et al./Polymer 41(2000)4933–49424934Fig.1.Variation of G H with time for PEN melt at 240ЊC at three different frequencies.with time indicates the possibility of chemical crosslinkingbetween polymer molecules.Both thermal degradation andchemical crosslinking show irreversibility in the rheologicalresponses.On the other hand,a reversible change in G H and h H with time at a constant frequency may be caused by changing in the physical state of the polymer melts.A typi-cal example of the physical change is the isothermal crystal-lization.As the crystallites grow to larger sized spheruliteswithin the PEN melt through nucleation and growth,thehomogeneous melt system changes to the heterogeneoussystem.Thus the G H and h H increase with the crystallization time.Figs.1and2show the variation of the G H and h H of PEN melt with time at240ЊC at three different shear rates(1,3, and5rad/s).At the early stage of experiment,both G H and h H are increased slowly,indicating an induction time for crystallization.The induction period is the stage when randomly entangled polymer chains transform to the regular aligned lattice.Because of topological obstruction of such entanglements,the polymer crystallization is extremely slow[16].However,an abrupt increase of both parameters follows in some minutes.This phenomenon can be ascribed to the formation of tiny crystals so-called crystallites prob-ably due to shear-induced crystallization.It can be easily imagined that the homogeneous PEN melt changes to a suspension system with proceeding crystallization,in which numerous crystallites are dispersed in the homo-geneous molten polymer matrix.The viscosity increases due to increasing the volume fraction of dispersed crystal-lites with progressing crystallization,which is also reportedby others[7,9,11,14,17].The ceiling value of G H is the same regardless of frequen-cies and temperatures whenﬁnishing crystallization asshown in Fig.1.On the other hand,the ceiling value of h H is gradually decreased with increasing the applied frequency as shown in Fig.2.This is attributable to pseudo-plasticity.That is,the heterogeneous system is expected toshow yield behavior[18].At low shear rates the hetero-geneous systems exhibit very high viscosity,and almostunbounded viscosity at zero shear rate.The viscosity,however,is rapidly decreased if the shear rate exceeds acritical value.Consequently,the ceiling viscosity at1rad/sis greater than at5rad/s.In addition,the ceiling value of h H shows a gradual decrease with time after having reached maximum as shown in Fig.2,which is more noticeable at the higher frequency.The gradual decrease of h H seems to result from the restructuring of the heterogeneous systems. That is,the viscosity is decreased with shearing on account of destruction of the orderedﬁller particle structure.The destruction of the pseudostructure ofﬁller particles is increased as shear rate is increased.Fig.2reﬂects this.It is also noted in Figs.1and2that the induction time forcrystallization is decreased as frequency is increased.Anapplication of shear stress to a polymer melt would giverise to two characteristic responses,orientation and slippageof polymer molecules.They are associated with theW.J.Yoon et al./Polymer41(2000)4933–49424935Fig.2.Variation of h H with time for PEN melt at240ЊC at three different frequencies.W.J.Yoon et al./Polymer41(2000)4933–49424936Fig.3.Variation of G H(A)and h H(B)for PEN melt at3rad/s at three different temperatures.W.J.Yoon et al./Polymer41(2000)4933–49424937Fig.4.DSC thermograms of PEN isothermally crystallized at(A)230ЊC,(B)240ЊC and(C)250ЊC at various frequencies.macroscopic phenomena of elasticity and ﬂow,respectively.That is,the oriented polymer molecule has fewer possible conformations than the unoriented one,which results in lower entropy.At the melting temperature,the free energy of the crystal equals the free energy of the melt as written by [3]T mD H f D S f H m ϪH cS m ϪS c1Hence,for an oriented melt,the ensuing reduction in entropy raises T m .Further,it increases the degree of super-cooling,accelerating the rate of crystallization.In general,higher shear rate gives better chance for orientation.Con-sequently,the induction time for cystallization is decreased with increasing shear rate.In Fig.3(A)and (B)shows that the annealing temperaturehas a profound effect on the nucleation and crystallization mechanism of PEN melts.The increase of G H and h H with annealing time represents the extent of crystallization of the melts with annealing time.Fig.3suggests that the number and growth rate of the nucleated crystallites is greater at 230ЊC than at 250ЊC.That is,both nucleation density and growth rate of crystallites are diminished with raising the annealing temperature.This stands to reason because the maximum rate of the homogeneous crystallization of PEN melts is observed in the vicinity of 215ЊC.The viscosity behavior of the PEN melt with crystalliza-tion in Fig.3may be accounted for by adopting the Mooney equation in a qualitative manner [19].ln h =h 1K E F 21ϪF 2=F m2W.J.Yoon et al./Polymer 41(2000)4933–49424938Fig.4.(continued )Table 1The values of T m H and T m HH of PEN with frequency (230,240and 250indicate temperature.(a)and (b)indicate v 0 a and v 0 b ;respectively (see Fig.6).1,3and 5indicate frequencyT m HT m HH T m HT m HH T m HT m HH PEN230(a)259.2272.1PEN240(a)266.7PEN250(a)270.1PEN230(b)264.1270.9PEN240(b)272.1PEN250(b)280.1PEN2301255.6270.3PEN2401259.3269.3PEN2501269.1PEN2303256.0270.0PEN2403261.9268.7PEN2503270.4PEN2305256.2269.7PEN2405263.6268.0PEN2505270.7F m true volume of fillerapparent volume occupied by the filler3 in which h is the viscosity of the suspension,h l is the viscosity of the suspending medium,f2is the volume frac-tion of theﬁller,f m is the maximum volume fraction that theﬁller can have,and K E is the Einstein coefﬁcient,whose value is known to be2.5for the dispersed sphericalﬁller.W.J.Yoon et al./Polymer41(2000)4933–49424939Fig.5.WAXD patterns of PEN isothermally crystallized at(A)230ЊC,(B)240ЊC and(C)250ЊC at various frequencies.Referring to the Mooney equation,the crystallization patterns of PEN melts at 230ЊC and at 250ЊC are distinc-tively different from each other.The Mooney equation predicts that the degree of increasing the suspension visc-osity with increasing f 2is greatly increased if the spheres form aggregates because the aggregation of spheres (spheru-litic crystallites or crystals in this study)increases the appar-ent ﬁller volume fraction.That is,the immobile portions (homogeneous molten PEN matrix in this study)caged by aggregated spheres also act as ﬁller portion.On this assump-tion,it may be suggested that an application of higher shear rate during isothermal crystallization tends to increase the heterogeneous crystallization characteristics.Hence,the higher nucleation density and higher growth rate of the nucleated crystallites is obtained at higher frequency,and the resultant is more abundant with less stable a -form crys-tals (this will be discussed later in detail).The melt endotherms of quiescently and shear-induced crystallized PEN were shown in Fig.4and the correspond-ing peak temperatures are listed in Table 1.The double melting endotherm behavior is displayed during heating the PEN sample in the DSC cell.In the melting process of the shear-induced crystallized PEN sample,three endother-mic peaks are identiﬁed;a broad endotherm,a low endotherm (T m H ),and a high endotherm (T m HH )as shown in Fig.4.The broad endotherm might be due to the thermal history during cooling and reheating,and both low and high endotherms are due to the melting of original lamella and recrystallized one,respectively.These results well coincide with the results of Zachman et al.[20]:(1)no change of crystal modiﬁcation is observed during DSC scanning;(2)the double melting behavior of PEN is due to the mechan-ism based on melting and recrystallization;(3)the b -form crystal has the T m higher than the a -form crystal by 2ЊC;and (4)the peaks of two forms of crystal are not separated in DSC thermogramsIn Fig.4(A)–(C)v 0rad =s indicates quiescent crystal-lization.(A)indicates that the PEN sample was crystallized at 230,240,and 250ЊC for the same time that required in the ARES experiments,and (B)expresses the PEN sample crys-tallized in an oil bath at the same temperature as in (A)for the time long enough to fully crystallize.Since the crystal-lization time in (A)is much shorter than in (B),an exother-mic peak is observed in the thermogram (A)at around 205ЊC.The v 0rad =s (b)curves in Fig.4(B)show a single melting peak.The T m shifts to higher temperature and peak width gets narrower as the crystallization time and tempera-ture are increased.This is attributable to the increased perfectness of the resultant crystal structure,which is observed more clearly when the sample is crystallized at higher temperature as can be seen in Fig.4(C).Only the b -form crystal exists when the sample is isothermally crys-tallized at 250ЊC after having melted at 300ЊC.This result matches well with the X-ray data.As mentioned the T m of the b -form crystal is higher than the a -form crystal by 2–4ЊC.It has been known that PEN has two different triclinic crystal structures.Buchner et al.reported that crystalW.J.Yoon et al./Polymer 41(2000)4933–49424940Fig.5.(continued )structures are inﬂuenced by both melting and isothermal crystallization temperature.They observed that the b -form crystal appeared mainly when PEN was isothermally crys-tallized above 230ЊC quiescently and the a -form crystal did below 230ЊC after having melted at 300ЊC [20].Fig.5presents WAXD patterns of PEN specimens shear-inducedcrystallized at (A)230ЊC,(B)240ЊC,(C)250ЊC at several frequencies.In Fig.5(A)the WAXD patterns for v 0rad =s shows diffraction peaks at 15.6and 23.3Њwhich correspond to (010)and (100)plane of the a -form crystal,respectively.The intensity of these peaks has a tendency to increase with increasing frequency.It means that the appli-cation of shear promotes the formation of the a -form crystal and the increase of frequency increases the content of the a -form crystal.In Fig.5(B)the (010)plane peak of the a -form crystal is smaller than that of the sample crystallized at 230ЊC in Fig.5(A)for v 0rad =s :However,the plane peak is increased with increasing frequency.In the case of the (100)plane peak,a shoulder appears at v 0rad =s :As frequency increases,the intensity of the plane peak standing for the a -form crystal shows tendency to increase.Particu-larly,for v 0rad =s in Fig.5(C)any plane peak of the a -form crystal is not observed,which is consistent with the results reported by Buchner et al.[20].The (010)and (100)plane peaks appear simultaneously,and keep on growing with increasing frequency.In addition,all diffraction peaks of Fig.5shift to lower angle when frequency is increased.This suggests that there is deformation in the crystal struc-tures as well.Thus,this X-ray trace of the sample is similar to those of Fig.5(A)and (B),suggesting similarity in the crystallization behavior at 230–250ЊC.In general,the b -form crystal is thermodynamically more stable but more difﬁcult to nucleate than the a -form crystal and the form of the crystal is largely determined by kinetic factors during crystallization such as the rate of nucleation and spherulite growth [21].In the case of the a -form crystal,one chain passes through the unit cell and the chains in the crystal are extended.In the case of the b -form crystal,however,four chains pass through the unit cell and the chains in the crystal are not completely extended.When the polymer is sheared,the number of crystallites increases with shear rate,representing faster nucleation.Wolkowicz [14]mentioned that the number of crystallites increased exponentially with time at all shear rates.Also,this can be conﬁrmed in Fig.6,which indicates that nuclea-tion becomes increasingly profuse with increasing frequency until the crystalline structure formed is no longer distinguishable with a microscope [3,22].Hence,the content of the a -form crystal in the speci-men increases with frequency because the a -form crys-tal is apt to nucleate due to fast nucleation.Consequently,the resultant a -form crystal is thermody-namically less stable than the b -form crystal because of much reduced entropy by molecular orientation under high shear force.References[1]Hill MJ,Keller A.J Macromol Sci (Phys)1969;B3(1):153.[2]Andrews EH.J Polym Sci 1966;A-2(4):663.[3]Haas TW,Maxwell B.Polym Eng Sci 1969;9:226.W.J.Yoon et al./Polymer 41(2000)4933–49424941(B)(A)(C)Fig.6.Polarizing optical micrographs of PEN crystallized at 240ЊC (A)v 0;(B)v 1;and (C)v 5:[4]Pennings AJ,van der Mark JMAA,Booj HC.kolloid Z v Z Polym1970;236:99.[5]Mackley MR,Keller A.Polymer1973;14:16.[6]Peterlin A.Polym Eng Sci1976;16:126.[7]Kobayashi K,Nagasawa T.J Macromol Sci(Phys)1970;B4:331.[8]Lagasse RR,Maxwell B.Polym Eng Sci1976;16:189.[9]Titomanlio G,Brucato V.Plastics Processing Society,The TenthAnnual Meeting,Akron,OH,1965,p.93.[10]Ulrich RD,Price FP.J Appl Polym Sci1976;14:401.[11]Eder G,Janeschizt-Kriehl H,Liedauer S.Progr Polym Sci1989;15:629.[12]Liedauer S,et al.Int Polym Proc VIII1993;3:236–44.[13]Moitzi J,Skallcky P.Polymer1993;34:3168.[14]Wolkowicz MD.J Polym Sci:Polym Symp1978;63:365.[15]Sherwood CH,Price FP,Stein RS.J Polym Sci;Polym Symp1977;63:77.[16]Imai M,et al.Phys.Rev.1995;B52:12696.[17]Kim JG,Park HJ,Lee JW.Korean J Rheol1997;4:174.[18]Carreau PJ,De Kee DCR,Chhabra RP.Rheology of polymericsystems,New York:Hanser,1997.[19]Nielsen LE.Polymer rheology,New York:Marcel Dekker,1977.[20]Buchner S,Wiswe D,Zachman HG.Polymer1989;30:480.[21]Zachman HG,Wiswe D,Riekel C.Macromol Chem Suppl1985;12:175.[22]Kim SP,Kim SC.Polym Eng Sci1993;33:83.W.J.Yoon et al./Polymer41(2000)4933–4942 4942。

雅思剑桥11高分范文

雅思剑桥11高分范文Unfortunately, as an AI language model, I am unable to provide specific content from copyrighted sources such as the Cambridge IELTS 11 book. However, I can help you generate a high-scoring IELTS essay by providing you with a structure and some guidance on how to approach the task.Title: "The Impact of Social Media on Society"Introduction:- Introduce the topic and state the importance of social media in today's society.- Present the thesis statement, conveying the overall impact of social media on individuals and communities.Body Paragraph 1: Positive Effects- Discuss the positive impacts of social media, such as fostering communication and networking.- Provide examples of how social media has facilitated connections between people worldwide.- Emphasize the role of social media in disseminating information and enhancing education.Body Paragraph 2: Negative Effects- Address the negative effects of social media, such as addiction and privacy concerns.- Discuss the impact of excessive social media use on mental and physical health.- Examine the potential manipulation of social media platforms for political purposes.Body Paragraph 3: Mitigating the Negative Effects- Suggest ways to mitigate the negative impact of social media.- Highlight the importance of digital literacy and critical thinking. - Explain the role of individuals, families, educational institutions, and governments in addressing the challenges posed by social media.Conclusion:- Summarize the main points discussed in the essay.- Reiterate the overall impact of social media on individuals and communities.- Call for a balanced approach towards social media usage, focusing on maximizing the benefits while minimizing the drawbacks.Remember to carefully analyze the essay question and adapt your response accordingly. Additionally, enhance your essay with relevant vocabulary, transitional phrases, and well-structured sentences. Practice time management to ensure you have enough time to revise and proofread your essay. Good luck with your IELTS preparation!。

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annual volatilities of spot prices were 163.91% for copper, 119.05% for chromium, 168.24% for uranium, 153.87% for zinc, and 44.46% for lead. These data show that increasingly more companies are being exposed to high risks in natural resources prices, implying that the stable supply of natural resources is very important for the national economy and for the survival of a company.
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C. Won / Energy 34 (2009) 1215–1224

pricing model which is both easy to apply and realistic in order to help ﬁrms’ investment decision-making.
There are two major competing procedures for evaluating risky projects in natural resources developments: one is decision analytic, based on traditional discounted cash ﬂow and stochastic dynamic programming (Fleten et al. [11], Smith and McCardle [27], Szklo et al. [28]) and the other is option pricing model (OPM) or contingent claims analysis, based on the no-arbitrage theory of ﬁnancial markets (Brennan and Schwartz [3], Ekern [9], Emhjellen and Alaouze [10], Laughton [18], Paddock et al. [22], Schwartz [25], Sezgen et al. [26]). In this paper, we develop a new pricing model of natural resources projects by applying contingent claim analysis in order to help ﬁrms evaluate the projects more accurately and easily. Our model is different from previous models in several respects. First, our model uses different assumptions about underlying variables and different methodology. For example, we treat both output prices and development costs as stochastic and derive a closed-form solution through Martingale framework. While most of the previous literature treats only output prices as stochastic (Brennan and Schwartz [3], Dixit [6], Gibson and Schwartz [14], Laughton [18], Paddock et al. [22], Pindyck [23], Schwartz [24, 25], Sezgen et al. [26]) and does costs as uncertain, we make our model more realistic by treating both output prices and development costs as stochastic processes, because development costs change over time depending upon the ﬂuctuations in materials and labor costs necessary for the developments. In addition, whereas we utilize Martingale principle to derive the pricing equation, other authors use different methods, such as an arbitrage argument through self-ﬁnancing (Brennan and Schwartz [3]), a dynamic decision tree (Smith and McCardle [27]), and a real option framework by considering various managerial ﬂexibility in the process of developments (Cortazar and Casassus [5], Fleten et al. [11], Laughton [18], Olsen and Stensland [21], Paddock et al. [22], Sezgen et al. [26], Szklo et al. [28]).
Keywords: Valuation Natural resources investment Contingent-claim Bituminous coal
abstract
As Brennan and Schwartz [Brennan M, Schwartz E. Evaluating natural resource investment. Journal of Business 1985;58:135–57] point out in their pioneering work, the valuation of natural resources projects is particularly difﬁcult due to the high degree of uncertainty in output prices of resources. In general, there are two competing procedures to evaluate risky projects in natural resources developments. One is decision analytic, based on traditional discounted cash ﬂow and stochastic dynamic programming [Fleten SE, Maribu KM, Wangensteen I. Optimal investment strategies in decentralized renewable power generation under uncertainty. Energy 2007;32:803–15; Smith J, McCardle K. Valuing oil properties: integrating option pricing and decision analysis approaches. Operations Research 1998;46(2):198–217; Szklo AS, Carneiro JTG, Machado G. Break-even price for upstream activities in Brazil: evaluation of the opportunity cost of oil production delay in a non-mature sedimentary production region. Energy 2008;33:589–600], and the other is contingent claims analysis, based on the no-arbitrage theory of ﬁnancial markets [Brennan M, Schwartz E. Evaluating natural resource investment. Journal of Business 1985;58:135–57; Emhjellen M, Alaouze CM. A comparison of discounted cash ﬂow and modern asset pricing methods – project selection and policy implications. Energy Policy 2003;31:1213–20; Laughton D. The management of ﬂexibility in the upstream petroleum industry. The Energy Journal 1998;19:83– 114; Paddock L, Siegel D, Smith J. Option valuation of claims on real assets: the case of offshore petroleum leases. Quarterly Journal of Economics 1988;103(3):479–508; Schwartz ES. Valuing long-term commodity assets. Journal of Energy Finance and Development 1998;3(2):85–99; Sezgen O, Goldman CA, Krishnarao P. Option value of electricity demand response. Energy 2007;32:108–19]. In this paper, we use the second approach to develop a new model, and the main contributions are providing a tractable and realistic means of incorporating the option value and optimal timing into the investment decision in natural resources and presenting an example that shows option and timing considerations to be important. We demonstrated the validity of the model using both numerical analysis and real data.