题目内容：
根据以下材料回答第 1～4 题：Passage Five
Although no one is certain why migration Occurs，there are several theories．One the-ory 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，spend the sum-mer，and then went south again as the ice advanced in winter．In time，the migration be-came a habit，and now，although the glaciers have disappeared，the habit continues。
Another theory proposed that the ancestral home of all modern birds was the tropics．When the region became overpopulated，many species were crowded north．During thesummer，there was plenty of food，but during the winter，scarcity forced them to returnto the tropics．
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 preparethem for migration．One scientist has been able to cause midwinter migrations by exposingbirds to artificial periods of daylight．He has concluded that changes occur in the bodies ofbirds due to seasonal changes in the length of daylight．
According to one theory，when the glaciers disappeared，birds ．
参考答案：
答案解析：