单项选择题
Although no one is Certain why migration occurs, there are several theories. One theory 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, spent the summer, and then went south again as the ice advanced in winter In time, the migration became a habit, and now, although the glaciers have disappeared, the habit continues.
According to the theory of photoperiodism, ()
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