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 Northeasten 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 of30 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 expeeted. 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】解析：解题依据是第二段开头两行"A study published recently in the journal Nature shows that airbome leaded gas emissions from the United States were the leading contributor to the high concentration of lead in the snow in Greenland"．(最近《自然》杂志发表的一项研究报告表明，美国排放含铅气体是造成格陵兰岛积雪中铅含量很高的主要原因)。据此D项"美国是北极积雪中铅污染的主要来源"应为正确答案。
单选题
Lead accumulation" worldwide decreased significantly after the use of unleaded gas in the U.S.______.
单选题
How did scientists discover the source of lead pollution in Greenland?
【正确答案】
D
【答案解析】解析：解题依据为第三段"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"．从中可以看出D项"通过比较不同国家使用的无铅汽油化学成分"应该是恰当的归纳。
单选题
The authors of the Ambio study have found that______.