单选题
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
【正确答案】
B
【答案解析】
[注释] 本题依据是第二段前两句，关键词语是overpopulated，意思是“数量过多”。
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