阅读理解
To extinguish(熄)different kinds of fires, several types of fire extinguishers(灭火器)have been invented．They must be ready for immediate use when fire breaks out．Most portable(手提式的)kinds operate for less than a minute, so they are useful only on small fires．The law requires ships, trains, buses and planes to carry extinguishers．
Since fuel, oxygen(氧气)and heat must be present for fire to exist, one or more of these things must be removed or reduced to extinguish a fire．If the heat is reduced by cooling the material below a certain temperature, the fire goes out．Water is the best cooling material because it is low in cost and easy to get．
Another method of extinguishing fire is by cutting off the oxygen．This is usually done by covering the fire with sand, steam or some other things．A blanket(毯子)may be used to cover a small fire．
A third method is called separation, which includes removing the fuel, or material easy to burn, from a fire, so that it can find no fuel．
The method that is used to put out a fire depends on the type of fire．Fires have been divided into three classes．Fires in wood, paper, cloth and the like are called C1ass A fires．These materials usually help keep the fire on．Such fires can be stopped most easily by cooling with water．
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