Taking away a city's rubbish is a big job．Every day tracks come into a city to collect it．Most rubbish is made up of things we can't eat or use．If we kept these things we would soon have a mountain of rubbish．

In some cities the rubbish is collected and taken outside of city to a dump．Often the city dump is placed where the ground is low or there is a big hole．The kitchen rubbish is broken into small pieces and sent into the sewagesystem．The sewage system takes away the used water from toilets，bathtubs and other places．

To keep mice and flies away，some earth is used to cover the newly dumped rubbish．Later，grass may be planted on the rubbish﹣filled land．Finally，a house or a school may be built there，and then you'd never know that this had once been an old rubbish dump．

In other cities the rubbish is burnt in special places．The fire burns everything but the

metal．Sometimes the metal can be used again in factories where things are made of metal．The food parts of rubbish are put in special piles where they slowly change into something called humus，which looks like black earth．It is rich with the kinds of things that feed plants and help make them grow．

51．What's the main idea of the article？ 　　

A．	To let us know taking away rubbish is a big job．
B．	To explain the need for rubbish collection．
C．	To introduce different ways to treat rubbish．
D．	To tell us to take useful things out of rubbish．

52．We can usually read the article in． 　　

A．	a science book
B．	a story book
C．	a history book
D．	a tour guide

53．What's the meaning of the word"sewage"in the article？ 　　

A．	吸尘
B．	洗涤
C．	安全
D．	排污

54．The food parts of rubbish can be used again． 　　

A．	to feed people
B．	to feed plants
C．	to build a house
D．	for us to eat

55．How should we do with a used metal box in the passage？ 　　

A．	By throwing it away．
B．	By putting it in earth．
C．	By burning it off．
D．	By reusing and recycling it．

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