20. Be careful when you open an unknown document. It might contain the deadly virus which could entirely ______ your computer system in a minute.

　 A. break down　　 　　B. crash　　 　　　　　C. block out　 　　　　D. dissolve

19. _______ you feel exhausted after the long tiring journey, you may as well take a good rest at home.

　 A. So that　　 　　　　B. In that　　 　　　　C. Now that　 　　　　D. In order that

18. _______ 　cars when you cross the busy street. Which of the following is not OK?

　 A. Watch out for 　　　B. Look out for 　　　C. Watch over　 　　　D. Be careful of

17. It is predicted that a comet will probably hit the earth in about 10 years and scientists all over the world are thinking about possible ways to ________it from happening. Which of the following is not OK?

　 A. protect　　 　　　　B. stop　 　　　　　　C. keep　　 　　　　　D. prevent

16. It is never harmful _____ you to be polite and gentle to others and ______, as is often the case, you can get what you want.

　　 A. to; thus　 　　　　B. for; thus　 　　　　　C. of; therefore　 　　　D. with; however

15. –Do you know what speed _______ for a rocket to escape from the pull or gravity of the earth to send a satellite into outer space?

　 ---No idea at all. I have never got the ______ of it.

　　 A. it needs; idea　 　　B. is taken; hang　 　　C. is needed; hang　 　　D. it takes; thought

14. The single invention of the computer _______ many new professions.

　　 A. gave a birth to　 　B. gave births to　 　　C. gave birth to　 　　　D. gave birth for

13. Oxygen usually ______ in the form of gas in nature but when made cold enough, it can also take the form of solid.

　　 A. remains　　 　　　B. stays　　 　　　　　B. exists　 　　　　　D. survives

12. The acid rain is one of the chain reactions that ______ by environmental pollution

　　 A. is come about　　 　B. is caused　 　　　　C. are given birth　 　D. are brought about

11. The parents have devoted themselves to bringing up their children and now it is time for the children, ______, to show their devotion to their parents.

　 A. in their turn　　 　　B. in their return　 　　　C. in return　 　　　　D. in their turns