单项选择题

This passage supports the belief that ______.

A．exact reasons for migration are not known
B．glaciers caused birds to migrate
C．the ancestral home of birds was the tropics
D．birds migrate because of changes in temperature

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热门试题

单项选择题

What is the meaning of the underlined word waterproof in the third paragraph

A．Covered with water.
B．Full of water.
C．Cleaned by water.
D．Not allowing water to go through.

单项选择题

Pollution in water is noticed______.

A．when poisons are poured into water
B．when a great many fish and birds die
C．when the first organisms are affected
D．as soon as the balance of nature is destroyed