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根据以下材料回答第 1～4 题：Passage Two
A greenhouse is a building made of glass，which is used for keeping plants warm when the outside temperature is low．In a similar way there are several gases in t he atmosphere which trap(捕捉)the heat produced by the sun and prevent it from escapin9．This is called the“greenhouse effect”and these gases are known as“greenhouse gases”．This is not simply air pollution，because without them the earth would be 30 degrees colder and human life would not exist．In other words，the“greenhouse effect”is a natural course which is to some degree helpful to us．
The Droblem is that we have been putting too many of these gases into the earth’satmosDhere by burning large quantities of coal and oil and by cutting down forests．The rapid increase in greenhouse gases is making the world warmer，the world’s temperature has already gone up by half a degree this century，and the sea level has risen by lo centime ters．If carbon dioxide(二氧化碳)in the atmosphere doubles，there will probably be a rise in the earth temrmrature of between land 4．It seems ca small increase，but it would be enough to causemajor changes in geography and agriculture．Large areas of the world would be flooded， and some areas would become dry and unable to produce crops．SO，it’s important for human beings to realize greenhouse effect and pay enough attention to it．
What is“green house used”for?
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