If there ______ no water or air on the earth, all living things would die.

1. A.
   is
2. B.
   were
3. C.
   are
4. D.
   will be

If there ______ no water or air on the earth, all living things would die.

A. is            B. were               C. are         D. will be

If there ______ no water or air on the earth, all living things would die.

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.