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  3. Lead deposits, which accumulated in soil and snow during the 1960’s and 70’s, were primarily the result of leaded gasoline emiss

Lead deposits, which accumulated in soil and snow during the 1960’s and 70’s, were primarily the result of leaded gasoline emiss

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问题      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 the lead sources. The dominant lead ratio found in Greenland snow matched that found ingasoline from the United States.
     In a study published in the journal Ambio, scientists found that 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 respond rapidly to reductions in atmospheric pollution, but that these findings should not be used as a license to pollute.
How did scientists discover the source of lead pollution in Greenland?
选项 A、By analyzing the data published in journals like Nature and Ambio.
B、By observing the lead accumulations in different parts of the arctic area.
C、By studying the chemical elements of soil and snow in Northeastern America.
D、By comparing the chemical compositions of leaded gasoline used in various countries.
答案D
解析 细节题。根据原文第三段第一句“…the ratios of the different forms of lead in the leaded gasolines…”可知，不同国家和地区的汽油中铅的含量不同；根据“…thus enabled scientists to differentiate the lead sources．”可知，专家可以通过汽油中的铅含量来判断此种铅来自于何地。显然，就是利用这种判断汽油中铅含量的方法得出格陵兰岛的铅污染主要来自美国的结论。所以D正确。原文中没有提到要分析Nature和Ambio中报告给出的数据，所以A不对；原文没有提及观察北极不同区域铅的累积情况，所以B不对；对美国东北部土质进行监测的目的是测量土壤中铅的含量，见第四段“…that lead levels in soil…decreased markedly since…”，说明C也不对。  
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