According to one of the theory, when glaciers disappeared, birds______. The author states that birds left the tropics because__

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问题 According to one of the theory, when glaciers disappeared, birds______.
The author states that birds left the tropics because______.

选项 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

答案B

解析 第二段谈到所有现代鸟的故乡都在热带。第二句when the region became overpopulated,many different sorts of birds crowded north,由此可见鸟之所以离开热带地区是因为那里的鸟太多了。
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