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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热门试题

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

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