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

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热门试题

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

The living things die because there is no ______ in the river or sea.

A．oxygen
B．poison
C．water
D．fish