阅读短文，做出选择。Read the passages and make your choice.

We know that there is air all around the world. And we could not breathe without air. Airplanes could not fly without air. They need air to lift their wings. Airplanes can not fly very high because as they go higher the air gets thinner. If we go far enough away from the earth, we find that there is no air.

What is the sky? Nothing. Where is it? It is all around the world. The sky is space. In this space there is nothing except the sun, the moon and all the stars.

The moon is about 384,000 kilometers away from the earth. An airplane cannot fly to the moon because the air reaches to an altitude of only 240 kilometers. Then there is no air. But there is something that can fly even when there is no air. It is a rocket.

How does a rocket fly? If you want to know, get a balloon and blow it up, until it is quite big. Do not tie up the neck o f the balloon. Let go! The balloon will fly off through the air very quickly. The air inside the balloon tries to get out. It reaches out through the neck of the balloon and this pushes the balloon through the air. It does not need wings like airplane.

【小题1】The sky is full of ________.

A．air

B．dust

C．stars

D．cloud

【小题2】---How far away can an airplane fly away the earth?
---Within ________.

A．420,000 kilometers

B．1,000 kilometers

C．240 kilometers

D．2,000 kilometers

【小题3】A rocket can fly ________ into the sky.

A．as high as an airplane	B．higher than an airplane
C．not so high as an airplane	D．as far as 240 kilometers

【小题4】A rocket flies like a ________.

A．plane

B．bird

C．kite

D．balloon

【小题5】Why can’t airplanes fly very high? Because ________.

A．they don’t have many wings to support themselves

B．the air there gets thinner, and they need air to lift their wings

C．man has never tried to drive them very high

D．they need balloons to pull them up into the sky