单项选择题

Scientists have not yet determined ______.

A．all of the effects of pollution on the human body
B．how pollution can be controlled successfully
C．when the atmosphere first became polluted
D．how to wash the black snow clean

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热门试题

单项选择题

We can conclude that ______.

A．civilization may be doomed if pollution is not controlled
B．pollution is more serious in Europe than it is in America
C．most people do not know that pollution is a serious problem
D．black snow is not so serious compared with white snow

单项选择题

Air pollution causes airplane accidents because ______.

A．pilots become ill
B．visibility is reduced
C．engines fail.
D．the air is too hot