题目

Before winter arrives, many kinds of birds start to fly south. People often call it migration. How much do you know about migration? Why do only some birds migrate? How do birds know when to go south? Can they find their way? Some birds fly south because they may die from hunger if they don't do that. The woodpecker(啄木鸟),for example, doesn't need to fly south because it can find something to eat in the bark(树皮) of the trees. But some birds eat grains(谷物). Their food disappears in winter, so they must fly south where food can still be found. But why don't they stay there forever? 用四舍五入法求出一个数的近似数是31.00，这个数最大是A.30.95B.31.04C.31.004D.30.995