阅读：Some radio singals were heard in 1967.They were coming from a point in the sky where there was unknown star

Questions 21 to 25 are based on the following passage:

Some radio singals were heard in 1967.They were coming from a point in the sky where there was unknown star.They were coming very regularly,too:about once a seco nd,if they were controlled by clock.

The scientists who heard the signals did not tell anybody else.They were rather afraid to tell in case they frightened people.The signals were coming from a very small body—no bigger,perhaps than the earth.Was that why no light could be seen from it？Or were the signals coming from a planet that belonged to some other star？There was no end to the questions,but the scientists kept the news secret.“Perhaps there are intelligent beings out there.”they thought,“who are trying to send messages to other planets,or to us？So the news was not given to the newspaper.Instead,the scientists studied the signals and searched for others like them...Well,all that happened in 1967 and 1968.Since then scientists have learnt more about those strange,regular,radio signals.And they have told the story,of course.

The signals do not come from a planet;they come from a new kind of star called a “pulsar””.About a hundred other pulsars have now been found,and most of them are very like the first one.

Pulsars are strong radio stars.They are the smallest but the heaviest stars we know at present.A handful of pulsar would weigh a few thousand tons.Their light—if they give much light—is too small for us to see.But we can be sure of this,no intelligent beings are living on them.

21. The radio signals discussed in this passage____.

A.were regular B.were controlled by a clock

C.were heard in 1967 only D.were secret messages 

22. The radio singals were sent by____.

A.a satellite 

B.a planet

C.a sky body which was unknown at that time

D.intelligent beings who were unknown at that time 

23. The scientists did not tell people about the signals because____.

A.the singals stood for secret messages

B.people would ask them too many questions

C.they did not want to frighten people

D.they stood for unimportant messages

24. A pulsar is____.

A. a small heavy star which sends out strong radio signals and cannot be seen 

B. a small heavy planet which sends out strong radio signals and cannot be seen 

C. a small heavy satellite which sends out strong radio signals and cannot be seen

D. a small intelligent being who sends out strong radio signals and cannot be seen

25. Which of the following is true？

A.One of the pulsars found by scientists sends radio signals.

B.Pulsar began to send radio singals in 1967.

C.Scientists have searched for pulsars for many years but found none.

D.Scientists have found many pulsars since 1967.