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

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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

单项选择题

It can be inferred from the passage that ______.

A．raising the world's temperature only a few degrees would not do much harm to the life on earth
B．lowering the world's temperature merely a few degrees would lead many major farming areas to disaster
C．almost no temperature variations have occurred over the past decade
D．the world's temperature will remain constant in the years to come