单项选择题

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.

According to one theory, when the glaciers disappeared, birds()

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