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  3. Although no one is certain why migration occurs, there are several theories. One theory is based upon the premise that prehistor

Although no one is certain why migration occurs, there are several theories. One theory is based upon the premise that prehistor

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问题     Although no one is certain why migration occurs, there are several theories. One theory is based upon the premise that prehistoric birds of the northen Hemisphere were forced south during the Ice Age, when glaciers covered large parts of Europe, Asia, and North America. As the glaciers melted, the birds came back to their homelands, spent the summer, and then went south again as the ice advanced in winter. In time, the migration became a habit, and now, although the glaciers have disappeared, the habit continues.
    Another theory proposes that the ancestral home of all modern birds was the tropics. When the region became overpopulated, many species were crowded north. During the summer, there was plenty of food, but during the winter, scarcity forced them to return to the tropics.
    A more recent theory, known as photoperiodism, suggests a relationship between increasing daylight and the stimulation of certain glands in the birds’ bodies that may prepare them for migration. One scientist has been able to cause midwinter migrations by exposing birds to artificial periods of daylight. He has concluded that changes occur in the bodies of birds due to seasonal changes in the length of daylight.
The author states that birds left the tropics because ______. ( )
选项 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
答案B
解析
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