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Many mammals live through the winter by hibernating（冬眠）. There is a nice problem of definition here. Most experts now agree that a mammal can be said to hibernate only if body temperature drops greatly and its whole metabolism—including respiration and heart rate—is much reduced. This is the case with animals like the woodchucks, hamsters, and hedgehogs, which hibernate in their burrows, and bats, which gather in caves. Bears, however, are not classed as hibernators. They pass most of the winter sleeping, but their body temperature drops only a few degrees, and they can become active without going through a slow process of warming up. Bears, relying on fat reserves to keep them going, even produce their young during this period.

Mammals are warm-blooded, that is, they have a means of keeping the temperature of their bodies quite constant despite the normal temperature changes of the outside world. Cold-blooded animals, fish, reptiles, insects, amphibians, also have a certain amount of control over their body temperature. They can warm up, if they are too cold, by sunning themselves, or cool off by getting into the shade. But it is hard for cold-blooded animals to keep warm in the winter. For any animal to be active, its body temperature must be above freezing. This is because the chemistry of life depends on water in a liquid state. If the body actually freezes, the whole system is disrupted and the animal dies. So cold-blooded animals in the north must either find some place to spend the winter where temperatures do not reach the freezing points, or develop a special resting stage in which the water content of the protoplasm（原生质）is much reduced. This will make its freezing point much lower than usual. Thus animals about to hibernate often dig down into the soil.

1．Most experts agree that true hibernation involves____.

A．sleeping through the winter

B．reduced heart rate and temperature

C．decrease in water content of the body

D．spending the winter in burrows or caves

2．Bears can pass most of the winter sleeping because they____.

A．are hibernators

B．have a low body temperature

C．have a sufficient reserve of fat

D．have a slow process of warming up

3．Cold-blooded animals are so called because they____.

A．can not keep their body temperature constant

B．can not warm up their body temperature in the winter

C．can cool themselves off by getting into a shade

D．are not active in the winter

4．It is implied that the body temperature of a cold-blooded animal____.

A．is lower than that of a warm-blooded animal

B．is always very cold

C．stays at a fairly constant level

D．changes in response to air temperature

5．The reduction of the water content of an animal's body protoplasm will enable the animal____.

A．to reduce its body temperature

B．to survive the freezing temperature in the winter

C．to dig down deep into the soil

D．to stay completely motionless

1．【答案】B

【精析】文章第一段第三句：“Most experts now agree that a mammal can be said to hibernate only if body temperature drops greatly and its whole metabolism—including respiration and heart rate—is much reduced.”其中涉及“body temperature drops greatly”和“metabolism”与选项中的“reduced heart rate and temperature”相对应，因此B项为正确答案。

2．【答案】C

【精析】从第一段最后一句“Bears, relying on fat reserves to keep them going, even produce their young during this period.”可知，C项是符合的，故选C项。

3．【答案】A

【精析】文章没有具体给出“cold-blooded animal”的定义，但给出了“warm-blooded animal”的定义：“Mammals are warm-blooded, that is, they have a means of keeping the temperature of their bodies quite constant despite the normal temperature changes of the outside world.”故可判断出“cold-blooded animal”的定义应与之相反，另外给出的“cold-blooded animal”的例子也可佐证其义，故正确答案为A项。

4．【答案】D

【精析】从文章第二段第三句“They can warm up, if they are too cold, by sunning themselves, or cool off by getting into the shade.”可知，冷血动物的体温是随外界变化的，因此D项为正确答案。

5．【答案】B

【精析】从文章第二段倒数第三句“So cold-blooded animals in the north must either find some place to spend the winter where temperatures do not reach the freezing points, or develop a special resting stage in which the water content of the protoplasm is much reduced.”可知，冷血动物要么找一个不低于零点的地方过冬，要么减少动物身体原生质的水分。由此可以判断，若减少动物身体原生质的水分，动物可以在低于零点的地方过冬。因此正确答案为B项。