阅读理解 Lead deposits, which accumulated in soil and snow during the 1960''s and 70''s, were primarily the result of leaded gasoline emissions originating in the United States. In the twenty years that the Clean Air Act has mandated unleaded gas use in the United States, the lead accumulation worldwide has decreased significantly. A study published recently in the journal Nature shows that air-borne leaded gas emissions from the United States were the leading contributor to the high concentration of lead in the snow in Greenland. The new study is a result of the continued research led by Dr. Charles Boutron, an expert on the impact of heavy metals on the environment at the National Center for Scientific Research in France. A study by Dr. Boutron published in 1991 showed that lead levels in arctic (北极的) snow were declining. In his new study, Dr. Boutron found the ratios of the different forms of lead in the leaded gasoline used in the United States were different from the ratios of European, Asian and Canadian gasolines and thus enabled scientists to differentiate (区分) the lead sources. The dominant lead ratio found in Greenland snow matched that found in gasoline from the United States. In a study published in the journal Ambio, scientists found that lead levels in soil in the Northeastern United States had decreased markedly since the introduction of unleaded gasoline. Many scientists had believed that the lead would stay in soil and snow for a longer period. The authors of the Ambio study examined samples of the upper layers of soil taken from the same sites of 30 forest floors in New England, New York and Pennsylvania in 1980 and in 1990. The forest environment processed and redistributed the lead faster than the scientists had expected. Scientists say both studies demonstrate that certain parts of the ecosystem (生态系统) respond rapidly to reductions in atmospheric pollution, but that these findings should not be used as a license to pollute.
单选题 The study published in the journal Nature indicates that ________.
【正确答案】 D
【答案解析】根据题干中的journal Nature查找到第2段首句，D项为其同义表达；A项、B项均与第1、2段尾句相悖。根据倒数第3段，许多科学家原以为土壤和雪中的铝将会残留更长的时间；他们没料到这些铅会这么快就被自然界吸收了，因此C项不对。
单选题 Lead accumulation worldwide decreased significantly after the use of unleaded gas in the US ________.
【正确答案】 B
【答案解析】根据第1段尾句，Clear Air Act是法案，mandate为“批准”，分别对应B项中的law和enforce。注意，本题题于是问全世界(worldwide)的情形，因此不能依据第4段的内容选D项 A项和C项全然与文段内容不符，甚至相反。
单选题 How did scientists discover the source of lead pollution in Greenland?
【正确答案】 D
【答案解析】根据Greenland查找到第3段，该段中提到了美国、加拿大和欧亚，首句中含有核心词leaded gasoline，而ratios of the different forms of lead对应D项中的compositions，故D项为该段的最佳概括。
单选题 The authors of the Ambio study have found that ________.
【正确答案】 A
【答案解析】根据Ambio查找到倒数第2段，尾句中的process“处理，清除”，对应A项中的get rid of。
单选题 It can be inferred from the last paragraph that scientists ________.
【正确答案】 C
【答案解析】题目询问：从最后一段可以推断出，科学家们________。原文说：“科学家们说，两项研究都表明，对于大气污染的减少，生态系统的某些部分迅速做出了反应，但这些发现不应当被用作污染的许可。”A项“科学家们对森林污染之谜迷惑不解”。这与原文不一致；实际上，两项研究都已经得出了明确结论。D项科学家们缺少治理铅污染的充分手段”。而原文已提到，只要减少大气污染，生态系统就可以做出反应，从而解决铅污染问题。B项“科学家们因无铅汽油的使用而感到宽心了”。原文“but”后的内容是本段句子结构的重点所在，它表明在科学家看来，依然有人在污染大气，所以提出了“不可把研究结论作为污染的许可”的警告，科学家们并没有“feel relieved”。事实上，正确答案应当是C项科学家们认为，铅污染依然是个问题”。