55．A．young tree             B．big tree                 C．old man                    D．line

（A）

No air means death．Although we cannot see it, there is air around us．So air is everything, and it gives life to every living thing．Bad air makes people ill．We must have fresh air to keep us healthy．

In the city there are a great many people and there are too many cars running on the roads．The gas which the cars spread (扩散) out is full of poison(毒物)．Every day we breathe it in again and again．This makes us feel sick．Besides so many cars, there are many factories too．From the chimneys of these factories, we get the smoke usually in black and grey color．This kind of the smoke, when it mixes the air, is dangerous to our health, it is because smoke contains (包含) many poisonous solid and gases．So in order to keep us healthy we have to go out to the countryside to breathe more fresh air．Or we can go up hills to get more fresh air．

54．A．again                  B．also                  C．too                  D．already

53．A．bear                       B．see                   C．ask                 D．answer

52．A．pulled                B．tied                 C．did                   D．made

51．A．straight               B．strong               C．tall                  D．young

50．A．other                B．another              C．the other             D．one

49．A．tall                        B．old                  C．short                D．young

48．A．of                     B．both                     C．among                D．between

47．A．his                  B．their               C．her                D．one's

46．A．playing with          B．talking with         C．talking about       D．talking to