单项选择题

Air pollution causes airplane accidents because ______.

A．pilots become iii
B．visibility is reduced
C．engines fail.
D．the air is too hot
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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
The author mentions people dying of air pollution in ______.
A．Illinois
B．Pennsylvania
C．New Jersey
D．Washington
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