下列短文中有十個空白，每個空白有四個選項。根據(jù)上下文要求選出最佳答案，并在答題紙上將相應的字母涂黑。

　　One type of solar collector is not very expensive.  It is usually    11   on the sunny side of the roof of a building. The sun is most _12    on this side,and the unit will    13    more heat from the sun. The unit has a     14 or level surface.  The surface is black because black collects more heat from the'sun.   15   ,there are pipes with water. The top is a piece of glass. The sun   16   through the glass and air inside. It heats the    17_ that is also inside.  A pump,a machine to   18   the water,stafls when water is warm enough. The hot water goes into a storage tank. This tank   19   or keeps the hot water. Then there is hot water to use for washing clothes or   20baths and showers.

　　11. A. pushed       B. pulled         C. pressed          D. put

　　12. A. direct       B. opposite       C. negative         D. steep

　　13. A. assemble     B. swallow        C. reflect          D. collect

　　14. A. broad        B. narrow         C. flat             D. short

　　15. A. Outside      B.lnside          C. Upside           D. Downside

　　16. A. shines       B. burns          C. heats            D. brightens

　　17. A.air           B.water           C.liquid            D.light

　　18. A. move         B. remove         C. obtain           D. maintain

　　19. A. stores       B. weighs         C. yields           D. traps

　　20. A. take         B. took           C. taking           D. takes