单项选择题

The author states that birds left the tropics because ______.

A．there was not enough food there in the winter
B．there were too many birds
C．there were too many glaciers
D．there was too much daylight
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单项选择题
According to one theory, when the glaciers disappeared, birds ______.
A．stopped migrating
B．continued migrating
C．stayed in the north
D．migrated south and stayed there
单项选择题
In July Guangzhou Baiyun International Airport resumed its international service to ______.
A．the whole world
B．only four southeastern nations
C．the Southeast Asian nations
D．all Asian nations
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