单项选择题

According to one theory, when the glaciers disappeared, birds ______.

A．stopped migrating
B．continued migrating
C．stayed in the north
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 was not enough food there in the winter
B．there were too many birds
C．there were too many glaciers
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．birds migrate because of changes in temperature
C．the ancestral home of all birds was the tropics
D．glaciers caused birds to migrate
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