According to one theory, when the glaciers disappeared, birds ______. According to the theory of photo-periodism, ______.

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问题 According to one theory, when the glaciers disappeared, birds ______.
According to the theory of photo-periodism, ______.

选项 A、birds should migrate in the middle of the winter
B、longer days cause changes in the bodies of birds
C、seasonal changes in the length of days do not affect migration
D、increasing daylight increases the distance of migration

答案B

解析 本题解答依据在最后一句changes occur in the bodies of birds due to seasonalchanges in the length of daylight,即"日照时间的季节性变化会造成鸟类身体生理变化",由此推断白天不论长还是短的变化都可能引起鸟类体内变化。故longer days cause changes in the bodies of birds正确。
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