Global warming is when the earth heats up，the temperature rises.It happens when greenhouse gases—carbon dioxide，water vapor，nitrous oxide(二氧化氮)，and methane(沼气) 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 very 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.Once the light is inside the car，it is trapped and the heat builds up，just like it does in the earth’s atmosphere.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 on the other hand it would be burning hot.We would not get the sun’s heat and light to make the night somewhat warm.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 earth can get unusually warmer，and many plants，animals，and people will die.They would die because there would be less food (plants like corn，wheat，and other vegetables and fruits)．This would happen because the plants would not be able to take in the heat.This would cause us to have less food to eat，but it would also limit the food that animals have.Gradually，people，plants，and animals would all die of hunger.

People are doing many things to try to stop global warming.One thing people are doing is carpooling.Carpooling is driving with someone to a place that you are both going to.This minimizes the amount of greenhouse gases put into the air by a car.

Another thing that people are doing is being more careful about leaving things turned on like the television，computer，and the lights.A lot of people are taking time away from the television，and instead，they are spending more time outdoors.This helps our planet out a lot.Now，more people are even riding buses，walking to school，and riding their bikes to lower the amount of greenhouse gases in the air.

Planting trees and recycling also help.If you recycle，less trash goes to the dump，and less trash gets burned.As a result，there are fewer greenhouse gases in our atmosphere.Although adults do many things to help stop global warming，kids can do just as much.

1.By the example given in Paragraph 1，the author wants to ________.

A．explain how greenhouse gases work

B．show his feeling in a car in hot summer

C．tell us cars are causes of greenhouse gases

D．say that there are greenhouse gases in every car

2.The underlined word “it” in Paragraph 2 refers to “________”．

A．light? B．heat

C．temperature? D．greenhouse effect

3.What is the main idea of Paragraph 3?

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

B．Human beings are likely to disappear from the earth.

C．Crops can’t take the heat if there is greenhouse effect.

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

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

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

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

C．What global warming is doing to the environment.

D．Who should be responsible for the environmental problems.

We experience different forms of the Sun’s energy every day. We can see its light and feel its warmth. The Sun is the major source of energy for our planet. It causes the evaporation (蒸发) of water from the oceans and lakes. Sunlight also provides the energy used by green plants to make their own food. These green plants then provide food for all organisms(生物) on the Earth.
Much of the energy that comes from the Sun never reaches the Earth’s surface. It is either reflected or absorbed by the gases in the upper atmosphere. Of the energy that reaches the lower atmosphere,30% is reflected by clouds or the Earth’s surface. The remaining 70% warms the surface of the planet, causes water to evaporate, and provides energy for the water cycle and weather. Only a tiny part, approximately 0.023%, is actually used by green plants to produce food.
Many gases found in the atmosphere actually reflect heat energy escaping from the Earth’s surface back to the Earth. These gases act like the glass of a greenhouse in that they allow energy from the Sun to enter but prevent energy from leaving. They are therefore called greenhouse gases.
When sunlight strikes an object, some of the energy is absorbed and some is reflected. The amount reflected depends on the surface. For example, you’ve probably noticed how bright snow is when sunlight falls on it. Snow reflects most of the energy from the Sun, so it contributes to the low temperatures of winter. Dark-coloured surfaces, such as dark soil or forest, absorb more energy and help warm the surrounding air.
【小题1】According to the passage, the root( 根本的) cause for weather changes on the Earth is         .

阅读理解

　　The people on the earth belong to many different countries. We speak many different languages and have different ideas. We do not look alike and we do not talk alike. But we are all alike in one way. We are all humans born on the earth. All of us live and die on this planet. We have never known any other home.

　　Men believed for thousands of years that the earth was the centre of everything. They believed that the sun and the stars moved around us. It was said that the sun was put in the sky to give light and heat to our earth.

　　The old ideas of men should not surprise us. Most of us only believe what we see. The earth looks like the centre of everything from where we stand. One discovery after another proved that our earth is only a very small part of the great universe. They proved that the earth is one little planet moving round the sun. We do not believe that we are the centre any more.

　　The troth surprised many people. But in 1540, Copernicus, the great Polish astronomer, found proof that the earth goes round the sun. And he wrote his discoveries in a book which was printed in a few weeks before he died.

1．Which do you think is the best title for this passage?

[　　]

A．Where Humans from?

B．Our Planet, the Earth

C．What is the Earth?

D．Have you known the Earth?

2．As humans, where are we alike?

[　　]

A．Each of us can speak different languages.

B．Each of us lives in the same way.

C．Each of us can speak and think .

D．Each of us comes from different families.

3．Which is the old idea of man?

[　　]

A．The history of the earth was as long as thousands of years.

B．The stars moved around the sun and the sun around the earth.

C．Our planet was the centre of the universe.

D．It was only the earth that had light and heat.

4．Why do we easily think the sun moves around the earth?

[　　]

A．It is our eyes that make such a mistake.

B．It is our brain that makes a wrong judgement.

C．We stand where we shouldn't stand.

D．We see what we can see.

5．The truth that the earth moves around the sun surprised many people, which means ________ .

[　　]

A．it was impossible for them to believe others

B．it was possible for them to believe only in themselves

C．it was difficult for them to change the scientists' ideas

D．it wasn't easy for them to change their ideas