The earth is round,（）.

A.It

B.As

C.That

D.What

_______ the moon moves round the earth is well known to all of us.

A. That

B. What

C. When

D. As

_____ the moon moves round the earth is well known to all of us .

A.What

B.That

C.Which

D.As

A.It

B.As

C.That

D.What

The Earth has a force that pulls things toward itself. We call this force gravity(地心引力). This is something we live with all the time, and we take it for granted and hardly ever think about it. But it is a most important factor in rocket operation and must be overcome if we are to get anywhere in space, or off the ground at all.

Take the throwing of a ball as an example. The harder the ball is thrown, the faster and higher it will go. What is the secret？ It's speed. If we could throw the ball hard enough it would go up and up forever and never come down. The speed at which it would have to be thrown to do this is known as escape speed. Of course, we cannot throw a ball hard enough because the speed required to escape completely from' the Earth's gravity is seven miles per second, or over twenty-five thousand miles per hour.

Once escape speed has been reached by a spacecraft(宇宙飞船), no further power is needed. A rocket aimed at the Moon, for instance, will "coast"(滑行) the rest of the way because the Earth's gravity cannot then pull it back, and there is no air resistance(阻力) in space to slow it down. This "coasting" is known as "free fall". That does not mean the rocket is falling down towards the Earth but that it is traveling freely in space without the aid of power, like a bicycle coasting downhill.

Free fall is an important feature of space travel: it would be impossible to carry enough fuel to provide powered flight all the time.

What is the most important factor for a rocket to escape from the Earth？

A．To travel as fast as it can.

B．To overcome the Earth gravity.

C．To get away from the air resistance.

D．To have strong power in its operation.

he outside temperature is low. 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 of 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 10 centimetres. 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 degrees. This may seem a small increase, 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.

1. The"greenhouse effect"means _____.

A. the way in which gases are used for keeping plants warm

B. a natural course which is some degree helpful to us

C. the way"greenhouse gases"trap heat on the earth

D. the whole course in which greenhouse gases prevent heat

2. If there were no greenhouse effect, _____.

A. no plants would grow on our planet

B. earth would be thirty degrees colder

C. man could not live on the earth

D. All of the above answers are correct

3. Which of the following is NOT true？ _____

A. Burning too much coal and oil produces lots of greenhouse gases.

B. It has become warmer on the earth now than in the past.

C. Gases put into the atmosphere now will affect the earth years later.

D. The temperature in a greenhouse is as high as that in the atmosphere.

4. Suppose the earth's temperature rose by 3 degrees, _____.

A. great harm would be done to mankind

B. the sea level would go by 10 centimetres

C. all the land in the world would be flooded

D. crops would be unable to grow on the earth

5. This passage mainly deals with _____.

A. the concept(概念)and harm of greenhouse effect

B. the relation between greenhouse gases and man

C. the concept and change of greenhouse effect

D. the effect of the rise of the earth's temperature

The problem of underground water is more serious because（）

[A] the underground water trickles away all the year round

[B] the underground water is hidden deep beneath the Earth

[C] the change of it must be controlled by the NASA's satellites

[D] the dramatic change of it might affect the gravitational field

and the space __21__ them.

Most of the stars are much bigger than the moon, but they look smaller __22__ they are far away from the earth. At night, the moon is high in the sky like a light. But the moon has no light of its own . The light of the moon __23__ the sun. The sun gives us light, heat and life. And the plants, animals and men will die without the sun. The earth is going round itself __24__. When the part of the earth turns to the sun, it is day. The __25__ part of the earth is night.

The earth is our homeland. We should love it.

21)、

A.because

B.comes from

C.between

D.opposite

E.all the time

22)、

A.because

B.comes from

C.between

D.opposite

E.all the time

23)、

A.because

B.comes from

C.between

D.opposite

E.all the time

24)、

A.because

B.comes from

C.between

D.opposite

E.all the time

25)、

A.because

B.comes from

C.between

D.opposite

E.all the time

We all __________________ radiation here on earth from the sun, from cosmic rays and from radioactive minerals.

[A] receive

[B] accept

[C] bring

[D] catch