A、When it breaks out. B、How it comes about. C、What kind it is. D、Where it takes place. C根据第16题分析可知，选项中的it指代的是火灾。由短文中提到的the metho

To extinguish different kinds of fires, several types of fire extinguishers have been invented. They must be ready for immediate use when fire breaks out. Most portable 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 and heat must be present in order 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. The cooling method is the most common way to put out a fire. 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 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 upon the type of fire. Fires have been grouped in three classes. Fires in wood, paper, cloth and the like are called Class A fires. These materials usually help keep the fire on. Such fires can be stopped most readily by cooling with water.
Questions 16 to 18 are based on the passage you have just heard.
16. Which method is used if we cover a small piece of burning wood with a basin in order to stop the fire?
17. What should we first be clear about in choosing how to put out a fire?
18. What would the speaker probably discuss in the paragraph that follows?