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Kip, nap ,doze ,forty winks ,shut-eye and snooze are all expressions we use to describe that mysterious state, sleep. We all do it ,we can’t get by without it and by the time we reach old age most of us have spent 20 years sleeping .Yet ,nobody knows why we do it.

Most scientists reckon that by resting our bodies, we allow time for essential maintenance work to be done .Any damage that there is can be put right more quickly if energy isn't being used up doing other things . Injured animals certainly spend more time asleep than usual while their wounds are healing. And quite a few illnesses make us feel drowsy so our body can get on with curing us.

Sleep is controlled by certain chemicals .These build up during the day , eventually reaching levels that make us tired. We can control the effects of these chemicals to some extent .Caffeine helps to keep us awake while alcohol and some medicines make us sleepy.

By using electrodes ,scientists are able to study what goes on in people’s heads while they sleep .They have discovered that when we first drop off everything slows down. The heart beats more slowly and our breathing becomes shallow. After about 90 minutes our eyes start to twitch and we go into what is called REM sleep. REM stands for Rapid Eye Movement ,and it’s a sign that we’ve started to dream .

You have dreams every night ,even if you don’t remember them.There are all sorts of theories about why we dream .One is that it gives the brain a chance to sort out the day’s activities , filling everything away in the right place. Another is that the brain gets bored while we’re asleep and organizes its own entertainment - a sort of late - night cinema !

A lot of people say they have to have eight hours ‘ sleep every night while others seem to manage on a lot less . One thing’s for sure , we all need some sleep and going without it can have some very strange effects .An American disc-jockey ,who stayed awake for 200 hours to raise money for charity ,thought things were bursting into flames all around him after 120 hours without sleep .He survived the ordeal but was depressed for three months afterwards.
51.Most scientists think we sleep because:
a.all our energy has been used up
b.our brains are tired and need to rest
c.our bodies contain too many chemicals 

52.What have scientists discovered about dreamming ?
a.Some people never dream
b.Most people dream for ninety minutes every night
c.People's eyes move when they are dreamming 
d.People dream for different reasons 

53.What does the writer say about the amount of sleep we need ?
a.Most people need eight hours sleep a night
b.Not everyone needs the same amount of sleep
c.People who don't get enough sleep get depressed
d.People who sleep too little start imagining things 

54.What does the writer say about people's and animals sleeping habits ?
a.People and animals have similar sleeping habits 
b.People would benefit from copying animal's sleeping habit
c.People need more sleep than animals 
d.Animals need more sleep than people

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E. Give the correct form of the words in the brackets to complete the sentences

1. Water pollution is the ………………of bodies of water such as watering places.  (contaminate).

2.       He does the experiences on  …………….  chamicals.                                    ( poision)

3. Sewage from households is also the ……………….. of the water pollution.               ( cause )

4. Some ………………….. waste can lead to the air and water contamination.              ( chemistry)

5.  ………………..  use the DNA banks to save endangered species.                ( Science)

6. Most of ………………… in the net isn’t true.                                          ( informat )