Plants don’t have legs to run or arms to throw a ball, but that doesn’t  21  that plants can’t 22at all． If you watch closely, you will see 23 plants bend their stems and flowers toward the sunlight every day． 24  are plants able to move   25 they don’t have bodies like animals do?

       Plants cells  26  chemicals called auxins（植物生长素） to   27  them to move． To find out how chemicals   28,  think of a plant outside in a garden． Because the sun  rise in the east in the morning, the   29 side of the plant stem is in the light． But the west side of the stem is in the _30 ． So the cells in the shaded side of the stem   31 to make auxins, which make that side of the stem grow  32  while the side of the stem   33  the sun grows much slower．  When the shaded side grows faster, it makes the whole stem  34  toward the sunlight, and  35  leaves and flowers on the stem face the 36  too． When the sun moves to the west in the afternoon, the   37  thing happens and the stem bends toward the light again．

       38   is a reason why the plant produces chemicals that make stems 39  the sun．  Leaves  40 sunlight to make food for the plant． So it is important for stem to get as much light as they can．

21．A． tell                   B． promise           C． show               D． mean

22．A． grow                B． go               C． move               D． stand

23．A． most                 B． few              C． tall                 D． short

24．A． why                 B． How              C． Where              D． When

25．A． because              B． although           C． if                  D． as soon as

26．A． make                B． accept            C． keep               D． send

27．A． push                B． bring             C． help                D． stop

28．A． form                B． work             C． flow               D． look at

29．A． west                B． bright            C． opposite            D east

30．A． shade                B． light              C． dark               D． garden

31．A． stop                 B． begin           C． continue             D． help

32．A． fast                 B． slowly            C． up                 D． badly

33．A． near                 B． toward           C． against             D． in

34．A． bend                B． fall               C． turn                D． walk

35．A． some              B． all                 C． few                D． no

36．A． stem               B． west              C． ground             D． sun

37．A． common           B． strange            C． same               D． different

38．A． This                B． That             C． It                 D． There

39．A． follow             B． circle             C． catch              D． pull

40．A． receive             B． have             C． use                D． reduce

Space is a dangerous place, not only because of meteors（流星） 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（紫外线的） penetrate（穿过） the 6 .Cosmic（宇宙的） 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 （雷姆）”.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 to where you 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．so                        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．unusual              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