题目内容：
根据以下材料回答第 1～4 题：Passage Four
People often speak of fire as though it were a living creature－It grows,dances，needs oxygen，feeds on whatever it can find，and then dies．And when a forest fire rages out of contr01．threatening human lives and homes，it must be fought like a“wild animal．”The fight is often desperate，since firefighters’best efforts may be dwarfed by the fury of a 1arge fire．But the fire’s own traits can be used against it．
The heated air above a fire rised in a pillar of smoke and burnt gases，pulling fresh air in from the sides to replace it．Firefighters use this fact when they“fight fire with fire．”They start a fire well in front of the one which they are fighting．Instead of traveling on in front of the huge fire，the smaller five is pulled back toward it by the updrafts of the larger blaze。As it travels back to meet the large fire，the smaller backfire burns away the fuel that the forest fire needs to survive．
Even when a backfire has been well set，however，the fire may still win the sturggle．The wind which the firefighters used to help them may now become their enemy．When the backfire meets the main fire，before both die for Lack of fuel，there is tremendous
flame，great heat and wild winds．A strong gust may blow the fire into the treetops beyond the area，giving the fire new fuel and a new life
This passage focuses on．
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