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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.
Lead accumulation worldwide decreased significantly after the use of unleaded gas in the U.S.
选项 A、was discouraged.
B、was enforced by law.
C、was prohibited by law.
D、was introduced.
答案B
解析 细节题。根据第一段末句：自从美国实行空气洁净条例后，20年间，全世界的铅沉积水平都大大降低。理解mandate的意思“强制执行”，同义转述，即实行空气洁净条例是立法通过的，所以B正确。无铅汽油的使用是被提倡的，所以A违背原意；无铅汽油的使用是被立法通过，而不是禁止使用，所以C违背原意；D不够具体明确，应该是通过立法的途径，而不单单是简单的引入。  
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