All over the earth’s surface is a layer of air which extends upwards for many miles. This air 1 the oxygen without which neither plants nor animals 2 live. Its movements, temperature and pressure 3 the weather, and it is a vehicle 4 the clouds of water vapour 5 condense and fall as rain. It forms a blanket which 6 us from the extreme heat of the sun during the day and 7 the extreme cold when the sun has 8 .

　　It is chiefly 9 air that sound travels, so that if there were no air we should 10 practically nothing.

　　The atmosphere is held 11 the earth’s surface by the gravitational pull of the earth—that is, it has weight. High up it is thin, 12 near the surface it is compressed by the 13 of air above, and is more dense. The weight of air pressing 14 each square inch of surface at sea—level is nearly 15 15 , which means that the total force 16 the skin of an average man is about 30,000 pounds. He is not 17 this because the pressure is equal in all directions and the pressure inside him is equal 18 that without, but should he go up in a balloon to a height at which the outside pressure is 19 he would suffer acutely. It is 20 this reason that the cabins of aeroplans are “pressurized”.

　　1. A. forms B. contains C. consists D. fills

　　2. A. would B. should C. could D. needed

　　3. A. determine B. choose C. make D. create

　　4. A. into B. near C. for D. above

　　5. A. what B. and C. but D. which

　　6. A. protects B. proves C. controls D. helps

　　7. A. about B. out C. off D. from

　　8. A. sit B. set C. sat D. settled

　　9. A. into B. up C. through D. along

　　10. A. know B. hear C. get D. observe

　　11. A. to B. before C. near D. across

　　12. A. for B. since C. because D. but

　　13. A. pressure B. space C. weight D. movement

　　14. A. in B. on C. for D. around

　　15. A. kilometer B. ponds C. miles D. pounds

　　16. A. inside B. into C. of D. on

　　17. A. aware of B. sure of C. afraid of D. delighted about

　　18. A. at B. to C. against D. after

　　19. A. more or less B. much like C. much less D. more than

　　20. A. for B. to C. from D. by

　　参考答案：BCACD ADBCB ADCBD DABCA