单项选择题

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
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单项选择题
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
单项选择题
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
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