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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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.
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
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