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 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.

　　21. 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

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