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  3. Although no one is certain why migration (迁移) of birds occurs, there are several theories. One theory is based upon the statemen

Although no one is certain why migration (迁移) of birds occurs, there are several theories. One theory is based upon the statemen

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问题     Although no one is certain why migration (迁移) of birds occurs, there are several theories. One theory is based upon the statement that prehistoric birds of the northern hemisphere (半球队) were forced south during the Ice Age, when ice covered large parts of Europe, Asia, and North America. As the ice of rivers melted, the birds came back to their homelands, spent the summer, and then went south again as the ice advanced in winter. Once the migration became a habit, and now, although the glaciers (冰河) has disappeared, the habit continues.
    Another theory proposes that the ancestral (祖先的) home of all modem birds was the tropics (热带). When the region became overpopulated, many different kinds of birds were crowded north. During the summer, there was plenty of food, but during the winter, short supply forced them to return to the tropics.
    A more recent theory, known as photoperiodism (光照反应), suggests a relationship between increasing daylight and the feeling of certain glands (腺) in the birds’ bodies that may prepare them for migration. One scientist has been able to cause midwinter migrations by making birds uncovered to unusual periods of light. He has concluded that changes occur in the bodies of birds due to seasonal changes in the length of daylight.
选项 A、began migrating again
B、stopped migrating
C、continued migrating
D、migrated south and stayed there
答案C
解析 第一段最后一句讲到迁移已经成为习惯，即使现在冰河不存在了，这个习惯却延续下来了。所以选[C]。
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