单项选择题
A．there was not enough food there in the winter B．there were too many birds C．there were too many glaciers D．there was too much daylight
A．stopped migrating B．continued migrating C．began migrating again D．migrated south and stay there
A．exact reasons for migration are not known B．birds migrate because of changes in temperature C．the ancestral home of all birds was the tropics D．glaciers caused birds to migrate