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 ________.
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
25. It can be inferred from the last paragraph that scientists ________.
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