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
The author states that birds left the tropics because______.
A．there were too many glaciers
B．there were too many birds
C．there was not enough food there in winter
D．there was too' much daylight
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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