阅读理解 Lead deposits, which accumulated in soil and snow during the 1960''s mid 7O''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 airborne 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 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 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 North-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 Am/do study examined samples of the upper layers of soft 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 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】根据第三段中的"The dominant lead ratio found in Greenland snow matched that found in gasoline from the United States．”就可推断出极地雪地铅污染主要来自美国。
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】根据第一段第二句话"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．”，便可知无铅汽油的使用是由法律强制实施的。
单选题 How did scientists discover the source of lead pollution in Greenland?
【正确答案】 D
【答案解析】文章第三段指出，欧洲、亚洲、加拿大和美国的含铅汽油的成分不一样，因此可以据此找出铅污染源。这就是说，可以通过分析各国含铅汽油的化学成分来确定格陵兰岛铅污染的来源。
单选题 The authors of the Ambio study have found that ______
【正确答案】 A
【答案解析】根据文章倒数第二段最后一句"The forest environment processed and redistributed the lead faster than the scientists had expected．”，可断定[A]正确。
单选题 It can be inferred from the last paragraph that scientists ______
【正确答案】 C
【答案解析】文章最后一段在指出“Certain parts of the ecosystem respond to reductions in atmospheric pollution"的同时，强调指出"these findings should not be used as a license to pollute"，由此我们可以推理出，空气污染还是一个有待解决的问题。故答案为[C]。