2. 考研
  3. Although no one is certain why migration occurs, there are several theories. One theory claims that prehistoric birds of the Nor

Although no one is certain why migration occurs, there are several theories. One theory claims that prehistoric birds of the Nor

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问题     Although no one is certain why migration occurs, there are several theories. One theory claims that prehistoric birds of the Northern Hemisphere were forced south during the Ice Age. 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 in spite of the disappearance of glaciers.
    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 suggests a relationship between increasing daylight and the stimulation of certain glands (腺) 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.
This passage supports the belief that______.
选项 A、exact reasons for migration are not known
B、birds migrate because of changes in temperature
C、the ancestral home of all birds was the tropics
D、glaciers caused birds to migrate
答案A
解析 本题考查应试者对作者态度的理解，文章客观地介绍了关于候鸟迁徙的观点，全篇除了首句“Although no one is certain why migration occurs, …”，其他地方并没有表露作者观点。所以只能从这儿得出结论：“exact reasons for migration are not known”。
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