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
It can be inferred from the last paragraph that scientists ______.
A．are puzzled by the mystery of forest pollution
B．feel relieved by the use of unleaded gasoline
C．still consider lead pollution a problem
D．lack sufficient means to combat lead pollution
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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