Global warming happens when greenhouse gases trap heat and light from the sun in the earth’s atmosphere, which increases the temperature. This is like when heat is trapped in a car. On a hot day, the car gets hotter when it is out in the parking lot. This is because the heat and light from the sun can get into the car, by going through the windows，but it can’t get back out. This is what the greenhouse effect does to the earth .The heat and light can get through the atmosphere, but it can’t get out. As a result，the temperature rises.
Sometimes the temperature can change in a way that helps us. The greenhouse effect makes the earth appropriate for people to live on. Without it，the earth would be freezing，or it would be burning hot. It would be freezing at night because we would not get the sun’s heat and light .During the day, especially during the summer, it would be burning because the sun would be up with no atmosphere to filter(过滤)it，so people，plants，and animals would be exposed to all the light and heat.
Although the greenhouse effect makes the earth able to have people living on it，if there gets to be too many gases，the each can get unusually warmer, and many plants，animals．and people will die. Plants would die because they would not be able to take the heat .This would cause us to have less food to eat,and it would also limit the food that animals have. With less food for the animals that we need to survive we would even have less food. Gradually,people，plants，and animals would all die of hunger．
People are doing many things to try to stop global warming .One thing is carpooling—driving with someone to a place that you are both going to. Another thing is being more careful about leaving electrical devices turned on. Now,more people are even riding buses or bikes to lower the amount of greenhouse gases in the air .Although adults do many things to help stop global warming,kids call do just as much.
By the example given in Paragraph l, the author wants to__________.

A．explain how global warming happens.

B．show his feeling in a car in hot summer.

C．tell us cars are causes of greenhouse gases.

D．say that I there are greenhouse gases in every car.

The underlined word‘it” in Paragraph 2 refers to__________.

A．light	B．greenhouse effect
C．temperature	D．heat

What is the effect of atmosphere?

A．It only traps the heat of the sun.

B．It does great harm to humans.

C．It makes the earth neither too hot nor too cold.

D．It prevents all the light from the sun shining on the earth.

What is the main idea of the third paragraph?

A．lf there is greenhouse effect,all the plants will die

B．Human beings are likely to disappear from the earth

C．Crops can ′t take in the heat lf there is greenhouse effect

D．Although greenhouse effect is necessary,too much of it will cause us to die.

What will the author most probably discuss in the paragraph that follows?

A．What kids can do to help stop global warming.

B．Who should be responsible for the environmental problems.

C．What global warming is doing to the environment.

D．Why cars and buses pollute the air a lot.

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