B. Because the air under the mud will keep the cold out and it is comfortable there.
C. Because they feel comfortable there and they can get food easily.
D. Because they have enough to eat there 。 （key： D）
6. We will know an animal is hibernating ifA. it touches cold and is breathing very slowlyB. it is sensitive to painC. its temperature drops to below zero centigradeD. it does not move （key： A）
7. Why can a hibernating animal manage to live through the winter？
A. Because it hardly makes any movement and breathes very slowly and it has enough fat stored in its body.
B. Because it has already stored supplies of fat in its body.
C. Because it will not use energy while sleeping since they never make any movement then.
D. Because it doesn’t breathe at all. （key： A）
8.Why does the bear stop hibernating on warm winter days？
A. Because the warmth makes it think the winter has gone.
B. Because it has to eat some food for the next long sleep.
C. Because it feels cold and wants to warm itself up on warm winter days.
D. Because it is going to hunt something on warm winter days. （key： A）
9. Why does a squirrel hide nuts before the beginning of winter？
A. Because it will eat a lot in winter to keep warm.
B. Because it is hard to find food in winter.
C. Because it needs to eat a lot to hibernate.
D. Because it needs nuts to fight against its enemies. （key： B）
10. We human beings are able to keep the cold out because.
A. we are warm-bloodedB. we are clever enough to build houses and to wear clothesC. we are cold-blooded， and we do not need to hibernateD. for human beings， hibernation will use more energy （key： A）
Step3：language points：本单元重点句型
（1）Hibernation is more than sleep.
（2）People who find hibernating animals asleep often think that they are dead.
（3）You can touch it， or even pull its tail， without causing it to move or wake up.
（4）Hibernating in that way， the animals can sleep all through the winter.
（5） The second is connected with the main use the body makes of food—to supply the energy for movements.
（6） We have seen that hibernating animal reduces movements to far below the ordinary level.
（7） The animal hardly makes any movement， hardly uses any energy， and hardly needs any food.
（8）Some animals， including some bears， only half-hibernate.
难句解析：
1．They go to sleep in all kinds of places.它们到各种各样的地方去睡觉。
go to sleep，fall asleep与sleep，这三个词（组）都有“睡着”之意。go to sleep意思是“睡着，去睡觉”；fall asleep意思是“睡着了，酣睡”，它们都表瞬间动作，如表持续性动作或延续性状态，要用动词sleep。e.g.
①I found it very difficult to go to sleep．我发现很难人睡。
②Soon he fell asleep，for he was very tired．他很累，很快就睡着了。
③we have slept（for） eight hours.我们已睡了八个小时。
2．They lead an active life which keeps up their normal body temperature even in very cold winter weather.
他们生活得很活跃，即使在严寒的冬天也能保持正常的体温。
习惯用法
（2）keep up保持，维持，继续eg．① Let s hope the sunny weather keeps up for Saturday s tennis match.
但愿星期六网球比赛时还是这样的好天气。
②We are having difficulty keeping up。our mortgage payments．我们难以继续支付分期偿还的抵押货款。
