单项选择题
A．birds should migrate in the middle of the winter B．longer days cause changes in the bodies of birds C．seasonal changes in the length of days do not affect migration D．increasing daylight increases the distance of migration
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．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