问题补充：
Fire can help people in many ways. But it can be very dangerous. Fire can heat water, warm houses, give light and cook. But fire can burn things, too. It can burn trees, houses, animals or people. Sometimes big fire can burn forests.
Nobody knows for sure how people began to use fire. But there are many interesting stories about the first time a man or a woman started a fire. One story from Australia tells about a man very, very long time ago. He went up to the sun by a rope (绳子) and brought fire down.
Today people know how to make a fire with matches (火柴). Children sometimes like to play with them. But matches can be very dangerous. One match can burn a piece of paper, and then it might burn a house. A small fire can become a big fire very fast. Fire kills many people every year. So you must be careful with matches. You should also learn to put fires. Fires need oxygen (氧气). Without oxygen they will die. Cover a fire with water, sand or sometimes with your coat. This keeps the air away from a fire and kills it. Be careful with fire, and it will help you. Be careless with fire, and it will burn you.
49.How did people begin to use fire? .
A. Not everybody knows how people began to use fire
B. Nobody knows how to make a fire
C. It is an Australian who started a fire
D. We are not sure how people began to use fire
50.Children mustn’t play with matches because .
A. matches burn paper B. it isn’t interesting
C. they can be dangerous D. they can burn a house
51.When you are going to put out a fire, you ,
A. must be careful B. should keep air away from it
C. must know it is dangerous D. should cover it with waterB
答案：
(答案→)B
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