单项选择题

This passage is primarily concerned with the ______.

A．greenhouse effect
B．burning of fossil fuels
C．potential effect of air pollution
D．likelihood of a new ice age
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单项选择题
As pointed out at the beginning of the passage, people used to think that air pollution ______.
A．caused widespread damage in the countryside
B．affected the entire eastern half of the United States
C．had damaged effect on health
D．existed merely in urban and industries areas
单项选择题
The word offset in the second paragraph could be replaced by ______.
A．slip into
B．make up for
C．set up
D．catch up with
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