【由北京市丰台区2014高考模拟改编】
阅读下列短文，从每题所给的A、B、C、D四个选项中，选出最佳选项。
Have you ever looked toward the sky on a fall day and witnessed a group of migrating birds? If so, you probably noted the V-shaped formation of the birds or the birds flying in a ball-like formation. Why do birds fly this way? Many theories have been developed to explain the formation patterns of different types of birds.
