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

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

单项选择题

It can be concluded that_______.

A．raising the world' s temperature only a few degrees would not do much harm to 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