练习17

Space is a dangerous place， not only because of meteors but also because of rays from the sun and other stars.

The atmosphere again acts __1__ our protective blanket on __2__ .Light gets through， and this is essential __3__ plants to make the food which we __4__ .Heat， __5__ ， makes our environments tolerable and some ultraviolet rays penetrate the __6__ .Cosmic rays of various kinds come __7__ the air from outer space，but __8__ quantities of radiation from the sun are screened off.__9__ men leave the atmosphere they are __10__ to this radiation;__11__ their spacesuits or the walls of their spacecraft， if they are inside，__12__ prevent a lot of radiation damage.

Radiation is the greatest known danger to explorers in __13__ .Doses of radiation are measured in __14__ called“rems ”.We all __15__ radiation here on earth from the sun，from cosmic rays and from radioactive minerals.The“ __16__”dose of radiation that we receive each year is about two millirems;it __17__ according toswheresyou live， and this is a very rough estimate.

Scientists have reason to think __18__ a man can __19__ far more radiation __20__ without being damaged;the figure of 60 rems has been agreed.

1.A.for B.with C.on D.as

2.A.stars B.sun C.earth D.space

3.A.with B.from C.under D.for

4.A.use B.live C.eat D.get

5.A.again B.also C.besides D.too

6.A.environment B.space C.atmosphere D.earth

7.A.across B.to C.from D.through

8.A.valid B.enormous C.various D.proper

9.A.As soon as B.As well as C.As much as D.As possible as

10.A.shown B.exposed C.faced D.covered

11.A.but B.because C.so D.so that

12.A.get B.make C.have D.do

13.A.earth B.atmosphere C.space D.environment

14.A.pieces B.units C.parts D.elements

15.A.receive B.accept C.bring D.catch

16.A.conventional B.common C.general D.normal

17.A.shifts B.converts C.modifies D.varies

18.A.what B.which C.that D.why

19.A.put up with B.keep up with C.come up with D.catch up with

20.A.from B.than C.as D.away