阅读表达（共5小题，每小题2分，满分10分）
阅读下面短文并回答问题，然后将答案写在横线上（请注意问题后的词数要求）
[1] A green house is a building made of glass which is used to keep warm when the outside temperature is low. In a similar way there are several gases in the atmosphere (mixture of gases that surrounds the earth) which trap 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.
[2] The problem is that in the last century and a half, we have been putting too many of these gases into the earth’s atmosphere by burning large quantities of coal and oil and by cutting down forest. The rapid increasing 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 10 centimeters. 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 1 and 4℃. This____________________, 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 are probably experiencing only now the effect of the gases put into the atmosphere before the 1960s. Since then, our use of these gases has greatly increased.
小题1:In this passage, what can we mainly learn?   (No more than 10 words)
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小题2:What is “greenhouse effect”? (No more than 15 words)
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小题3:Why are greenhouse gases rapidly increased according to the text? (No more than 20 words)
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小题4:Fill in the blank in the second paragraph with proper words (no more than 6 words)
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小题5:What does “then” refer to in the last sentence? (No more than 2 words)
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