In a study published in the journal Ambio, scientists found that lead levels in soil in the North-eastern 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.

　　62. The study published in the journal Nature indicates that ________.

　　A) the Clean Air Act has not produced the desired results

　　B) lead deposits in arctic snow are on the increase

　　C) lead will stay in soil and snow longer than expected

　　D) the US is the major source of lead pollution in arctic snow

　　63. Lead accumulation worldwide decreased significantly after the use of unleaded gas in the US ________.

　　A) was discouraged C) was prohibited by law

　　B) was enforced by law D) was introduced

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