题目内容：
完成选项 A A greenhouse is a building made of glass which is used for keeping plants warm when the outside temperature is how．In a similar way there are several gases in the atmosphere which trap(捕捉)the heat produced by the sun and prevent it from escaping．These gases are known as“greenhouse gases“，and the way in which they trap heat in the atmosphere is called the “greenhouse effect”．This is not simply air pollution．Most of the main greenhouse gases exist naturally in small amounts in our atmosphere．And without them 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 problem is that in the last century and a half．we have been putting too many 0f these gases into the earth’s atmosphere 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 centi-metres．If the amount of carbon dioxide(二氧化碳)in the atmosphere doubles，there will probably be a rise in the earth’s temperature of between l and 4 degrees．This may seem a small in—crease，but it would be enough to cause major changes in geography and agriculture．Large areas of the world would be flooded．and some areas would become dry and unable to produce crops．It is important，too，to consider that there may be a delay of about 30 years in the greenhouse effect．This means that we’re probably experiencing only now the effect of the gases put into the atmosphere before the l960s．Since then，our use of these gases has greatly increased．
“Greenhouse effect”means________． A．a natural course which is to some degree helpful to us
B．the way．in which gases are used for keeping plants warm
C．the way“greenhouse gases”trap heat on the earth
D．the whole course in which greenhouse gases prevent heat
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