Passage 3 　　Questions 31 to 35 are based on the following passage: 　　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 bum things, too.It can burn trees, houses, animals or people.Sometimes big fires 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 out 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.

Which of the following is the main idea of this passage

A．Fire can help people in many ways
B．Fire can be both helpful and dangerous
C．Fire can burn things and people
D．We must be careful with matches