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2016高考英语阅读理解和短文改错选练（6） 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. But saving energy is important for more than one reason. Sometimes food is short during migration flights. Keeping energy enables the birds to fly longer distances between meals. When food is sighted, the birds guide one another in a different way. When a bird identifies a familiar feeding area, it might turn around in order to signal the group to change direction. Then, this bird becomes the new leader. It helps guarantee that other birds will know exactly where it is going. Then the whole group makes a change in direction, gently streaming from the sky down to the ground. This formation is like an arrow pointing to the location of food. | 
