单项选择题

According to one of the theory, when glaciers disappeared, birds______.

A．began migrating again
B．stopped migrating
C．continued migrating
D．migrated south and stayed there
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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
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.
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