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
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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
单项选择题
Why did one scientist expose birds to artificial daylight
A．To test the relationship between daylight and a disease of the glands common to birds.
B．To test the relationship between daylight and migration.
C．To test the relationship between migration and temperature.
D．To test the relationship between daylight and changes in the season.
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