Cloze Test(10 points,1 point each)

下列短文中有十个空白，每个空白有四个选项。根据上下文要求选出最佳答案，并在答题纸上将相应的字母涂黑。

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.