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
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单项选择题
According to the theory of the photoperiodism,______.
A．birds should migrate in the middle of the winter
B．there were too many glaciers
C．longer days cause changes in the bodies of birds
D．increasing daylight increases the distance of migration
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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