Lead deposits, which accumulated in soil and snow during the 1960’s mid 7O’s,

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问题 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. [br] Lead accumulation worldwide decreased significantly after the use of unleaded gas in the US______

选项 A、was discouraged
B、was enforced by law
C、was prohibited by law
D、was introduced

答案 B

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。

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