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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热门试题

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

单项选择题

The way to stop water pollution is______.

A．to put oxygen into the river
B．to realize the serious situation clearly
C．to make the waste materials harmless
D．to make special room in the sea for our waste