题目

设函数y=-x2+l的切线l与x轴，y轴的交点分别为A，B，O为坐标原点，则△OAB的面积的最小值为__________. 答案：Some mammals migrate（迁徒）. One of these is the caribou（驯鹿）. They travel to the tundra（苔原）every summer to eat the rich grasses that grow there. When the weather becomes cooler, they migrate to the warmer forests and spend the winter there. Another mammal that migrates is the humpback whale. They spend the winter in Hawaii, and the summer in the Arctic（北极的）waters. They travel to the Arctic to eat millions of small shrimp（虾）.Anther habit that some animals have to survive the cold winter is called hibernation. Animals that hibernate, rest or sleep during the winter. While they are hibernating, their bodies use up the fat layer that they have put on by eating large amounts of food during the summer. Some animals that hibernate in the Arctic are small animals called lemmings（旅鼠）, squirrels（松鼠）and bears.There are two habit adaptations that animals in the Arctic have all year round. These are called herding and pack life. Caribou are example of animals that live in herds. A herd is a group of animals that lives together for most, or all of their lives. A herd of caribou can have a thousand or more animals in it. They live in herds for protection. It is easier for caribou to protect themselves when they are together than it is when they are alone. This is because the meat-eaters hunt in packs.A pack is a small group of animals that lives together. It is usually controlled by a head male and a female. All the members of the pack must obey them. Packs have a very definite social structure. Each animal in a pack has a different position, or importance. Animals in a pack are usually meat-eaters. They live together for protection and hunting purposes.【小题1】According to the first paragraph, the humpback whale         .A．travels to the Arctic for shrimp in summerB．spends the winter in the ArcticC．lives in Hawaii in summerD．has the same living habits as the caribou【小题2】What kind of animal doesn’t hibernate in the Arctic?A．The caribou.B．The bear.C．The squirrel.D．The lemming.【小题3】 Caribou live in herds in order to         .A．use up their fat layerB．find foodC．protect themselvesD．hunt in packs【小题4】 What do we know about pack life according to the last paragraph?A．Animals in pack are usually grain-eaters.B．All animals are in an equal position in a pack.C．It is either controlled by a head mate or by a female.D．Each animal has its own different role in a pack.