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  3. No one knows for sure why birds migrate (定期迁徙). One theory of migration says that ancient birds of the northern part of the eart

No one knows for sure why birds migrate (定期迁徙). One theory of migration says that ancient birds of the northern part of the eart

admin2012-08-21  43
问题     No one knows for sure why birds migrate (定期迁徙). One theory of migration says that ancient birds of the northern part of the earth were forced southward during the Ice Age, when ice covered large parts of Europe, Asia and North America. As the ice melted, the birds came back to their homelands, spent the summer, and then went south again in winter. Gradually, these comings and goings became habits, and birds now migrate though much of the ice has gone.
    Another theory proposes that the ancient home of all modern birds was the tropic areas. There they lived so well that the region became overcrowded. Many species had to move northward. During the summer, these birds found plenty of room and food. In winter, however, food became scarce and they had to return to the south.
    A newer theory is that increasing daylight stimulates certain glands (腺) in the bird’s body and prepares it for migration. One scientist is able to make birds migrate in midwinter by exposing them for two months to artificial daylight. Recoveries of marked birds indicate that they fly north as soon as they are set free. The conclusion is that the urge to migrate is determined by changes in the bird’s body which take place under seasonal changes in the length of daylight.
    This theory would account for the fact that not all birds migrate at the same time. Each species seems to have its own schedule. The theory would also account for the regular time and routes of migration. Birds arrive at a given place year after year. Unfavorable weather delays them only a few days. Total hours of daylight, rather than weather, start them on the way.
    How birds find their way to the same place year after year and why they follow their own particular route are still mysteries. They evidently do not follow known landmarks (地 面标志), for many young birds migrate alone without the help of experienced adults.  
Which of the following can NOT be inferred from the passage?
选项 A、Birds do not migrate at the same time.
B、Birds usually follow regular routes for migration.
C、Each species has a special reason for migration.
D、Birds migrate to their own particular places every year.
答案C
解析 文章提到了鸟类不同时迁徙，即选项A；也提到了鸟类迁徙的路线和迁徙地点(最后一段第一句话)，即选项B和D。而只有C选项的内容，即每个物种都有一个特殊的迁徙原因，从原文中推断不出来。
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