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阅读下面的文字，完成文后五题。人们对碳正离子进行研究时发现，它的“寿命”非常短，大约在10-23～10-6秒之间，因此，不能直接用仪器观测到这个有机反应中重要的中间体的“面目”。欧拉的贡献在于他发现了制备“长寿”高浓度碳正离子的方法。他采用了亲和能力小的溶剂，在低温(大约-100℃)下，可以使碳正离子具有长寿命。为了得到高浓度的碳正离子，欧拉使用一种很强的酸(被称为路易斯酸)，它可以使卤代烷生成碳正离子。欧拉的成就是建立在许多科学家辛勤工作的基础之上的。早在20世纪50年代，温斯坦在研究亲和取代反应和一些有机物重排时曾经假设，中间体碳正离子是非经典的，它包含一个5配位价的碳。1979年的诺贝尔化学奖获得者布朗对此也做过研究。但是这些仅是假设和解释而已。欧拉的大量工作证明了在强酸介质中，饱和碳氢化合物会发生质子化。通过对碳正离子的分析，能够发现低成本制造化工产品的方法，目前已有几十种产品得益于欧拉的发现。 下列对第三段内容概括正确的一项是（）。
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Within hours of appearing on television to announce the end of conscription, President Jacques Chirac moved quickly to prevent any dissent from within the military establishment. Addressing more than 500 military staff officers at the military academy in Paris yesterday, Mr. Chirac said clearly that he "expected" their loyalty in the work of rebuilding France’s national defense.He understood their "legitimate concerns, questions and emotions" at the reforms, but added. "You must understand that there is not and never has been any rigid model for French defense. Military service has been compulsory for less than a century. Realism required that our armed forces should now be professional."The president’s decision to abolish conscription over a period of six years removes a rite of passage for young Frenchmen that has existed since the Revolution, even though obligatory national service only became law in 1905. As recently as 1993, an opinion poll showed that more than 60% of French people said they feared the abolition of conscription could endanger national security. A poll conducted this month, however, showed that 70% of those asked favored ending of practice, and on the streets and in offices yesterday, the response to Mr. Chirac’s announcement was generally positive.Among people who completed their 10-month period of national service in the last few years or were contemplating the prospect, there was almost universal approval, tempered by a sense that something hard to define--mixing with people from other backgrounds, a formative experience, a process that encouraged national or social cohesion--might be lost.Patrick, who spent his year in the French city of Valance assigning and collecting uniforms, and is now a computer manager, said he was in tears for his first week, and hated most of his time. He thought it was "useless" as a form of military training-- "I only fired a rifle twice"--but, in retrospect, useful for learning how to get on with people and instilling patriotism.As many as 25% of those liable for military service in France somehow avoid it--the percentage is probably much greater in the more educated and higher social classes.According to Geoffroy, a 26-year-old reporter, who spent his time in the navy with the information office in central Paris, the injustice is a good reason for abolishing it. People with money or connections, he said, can get well-paid assignments abroad. "It’s not fair: some do it, some don’t."Several expressed support for the idea of a new socially-oriented voluntary service that would be open to both men and women. But the idea seemed less popular among women. At present, women have the option of voluntary service and a small number choose to take it. We learn from the passage that French women （）
数据模型按不同应用层次分成3种类型，它们是概念数据模型、【 】和物理数据模型。
Many theories concerning the causes of juvenile delinquency (crimes committed by young people) focus either on the individual or on society as the major contributing influence. Theories (21) on the individual suggest that children engage in criminal behavior (22) they were not sufficiently penalized for previous misdeeds or that they have learned criminal behavior through (23) with others. Theories focusing on the role of society suggest that children commit crimes in (24) to their failure to rise above their socioeconomic status, (25) as a rejection of middle-class values. Most theories of juvenile delinquency have focused on children from disadvantaged families, (26) the fact that children from wealthy homes also commit crimes. The latter may commit crimes (27) lack of adequate pa- rental control. All theories, however, are tentative and are (28) to criticism. Changes in the social structure may indirectly (29) juvenile crime rates. For example, changes in the economy that (30) to fewer job opportunities for youth and rising unemployment (31) make gainful employment increasingly difficult to obtain. The resulting discontent may in (32) lead more youths into criminal-behavior. Families have also (33) changes these years. More families consist of one-parent households or two working parents; (34) , children are likely to have less supervision at home (35) was common m the traditional family structure. 30（）
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