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Passage One

Questions 1 to 5 are based on the following passage.

  • Although no one is certain why migration occurs, there are several theories. One theory is based upon the premise that prehistoric birds of the Northern Hemisphere were forced south during the Ice Age, when glaciers covered large parts of Europe, Asia, an
  • 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
  • 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 b
  • According to one theory, when the glaciers disappeared, birds_______.
  • A.stopped migrating
  • B.continued migrating
  • C.began migrating again
  • D.migrated south and stayed there
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