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
According to the theory of photo-periodism, ______.
A．birds should migrate in the middle of the winter
B．longer days cause changes in the bodies of birds
C．seasonal changes in the length of days do not affect migration
D．increasing daylight increases the distance of migration
单项选择题
Those students don't have to be made ______ . They are all diligent students.
A．learn
B．learned
C．learning
D．to learn
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