单项选择题

The amount of oxygen an animal uses depends on ______.

A．the length of time it lives
B．the food it eats
C．its size and shape
D．the heat the body produces
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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
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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