Scientists say both studies demonstrate that certain parts of the ecosystem (生态系统)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
C) lead will stay in soil and snow longer than expected
D) the US is the major source of lead pollution in arctic snow
22.Lead accumulation worldwide decreased significantly after the use of unleaded gas in the US ________.
A) was discouraged
B) was enforced by law
C) was prohibited by law
D) was introduced
23.How did scientists discover the source of lead pollution in Greenland?
A) By analyzing the data published in journals like Nature and Ambio.
B) By observing the lead accumulations in different parts of the arctic area.
C) By studying the chemical elements of soil and snow in Northeastern America.
D) By comparing the chemical compositions of leaded gasoline used in various countries.
24.The authors of the Ambio study have found that ________.
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