题目内容：
根据下列材料回答 题。Passage Two
Although no one is certain why migration occurs，there are several theories．One theory isbased upon the premise that prehistoric birds of the northen Hemisphere were forced southDur-ing the Ice Age，when glaciers covered large parts of Europe，Asia，and North America．As theglaciers melted，the birds came back to their homelands，spent the summer，and then went southagain as the ice advanced in winter．In time，the migration became a habit，and now，althoughthe glaciers haveDisappeared，the habit continues．
Another theory proposes that the ancestral home of all modem birds was the tropics．Whenthe region became overpopulated，many species were crowded north．During the summer，therewas plenty of food，but During the winter，scarcity forced them to retum to the tropics．
A more recent theory，known as photoperiodism，suggests a relationship between increasingday light and the stimulation of certain glands in the birds’bodies that may prepare them for mi—gration．One scientist has been able to cause midwinter migrations by exposing birds to artificialperiods of Daylight．He has concluded that changes occur in the bodies of birdsDue to seasonalchanges in the length of Day light．
According to one theory，when the glaciers Disappeared，birds_________． A．stopped migrating
B．continued migrating
C．began migrating again
D．migrated south and stay there
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