[单项选择] 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
A. forests get rid of lead pollution faster than expected
B. lead accumulations in forests are more difficult to deal with
C. lead deposits are widely distributed in the forests of the US
D. the upper layers of soil in forests are easily polluted by lead emissions

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