答：活塞跟筒壁紧密接触可以上下滑动，A、B是只能向上开启的阀门．使用时，提起活塞时，阀门A受到____的作用而关闭，活塞下面的空气变稀薄，气压____（填“大于”或“小于”）外界大气压，于是，低处的水在大气压的作用下推开阀门____进入圆筒．当压下活塞时，阀门B受水的压力而关闭，水不能向下流，于是就冲开阀门____进入圆筒的上部．再提起活塞时，活塞上面的水使阀门A关闭，水被活塞提起，从侧管中流出来．

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    What do you do on weekends? Some people like to stay at home, but others like to leave home and do other things. My friend, Tom works hard in a factory. He has a son and a daughter.

    On weekends, he usually does the same things. On Saturdays he washes his car and clothes. On Sundays, he goes to a farm with his family by car. His parents live there. The farm isn't very big, but there're often many things to do on the farm. The children help feed (饲养) the animals. There are many animals on the farm. You can see sheep, cows, horses and pigs. Tom and his wife help in the fields. At the end of the day, children are very tired and hungry. Tom's mother gives them a big meal. They are very happy.