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

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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

单项选择题

This passage supports the belief that ______.

A．exact reasons for migration are not known
B．glaciers caused birds to migrate
C．the ancestral home of birds was the tropics
D．birds migrate because of changes in temperature