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 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
单项选择题
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 dements of soil and snow in Northeastern America.
D．By comparing the chemical compositions of leaded gasoline used in various countries.
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