单项选择题

Many scientists hold that global warming can be controlled by.

A．reducing the polar bear population.
B．paying more attention to the earth's rising temperatures
C．reducing carbon dioxide emissions
D．refreezing the polar ice caps

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单项选择题

By saying Spring is usually prime food time for 1,200 polar bears (Line I, Para. 1), the author means that ______.

A．spring is usually a good time for polar bears to carry out their mating rituals
B．the polar bears usually eat a lot in the spring
C．spring is generally a good time to hunt polar bears
D．polar bears usually hibernate in the spring

单项选择题

Which of the following is listed as being evidence for global warming

A．Water shortages in cities.
B．Earlier thawing of lakes and rivers.
C．Increased levels of greenhouse gases.
D．The rapid extinction of the polar bear.