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.
One theory is that birds fly in certain formations to take advantage of the laws of nature. The birds know that flying in a V-shaped pattern will save energy. Like the lead cyclist in a race who decreases wind force for the cyclists who follow, the lead bird cuts wind force for the birds that follow. This decrease in wind force means that the birds use up to 70 percent less energy during their flight. When the lead bird becomes tired, a more rested bird takes over that position.
