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If someone ask you, "What colour is the sky?"I think you will answer "Blue".I'm afraid that you will be
wrong. The sky has no colour. It looks blue because we are looking at blue sunlight. The sunlight is
shining on little bits of dust ( 灰尘) in the air.
Is the sky full of air? I'm sure that you have asked this question, too. We know that there is air all
around the world, but we couldn't fly very high because as we go higher, the air gets thinner. If we go far
enough away from the earth, we find there is no air.
Maybe we can answer some questions now. What is the sky? Where is it? It is all round the world. The
sky is space (宇宙空间). In space there is nothing, but only the sun, the moon, and all the stars.
1. We often think the sky is blue because _____.
A. we think it is so
B. we like this colour
C. we don't have other colours
D. we don't like other colours
2. The sky is _____.
A. a star round it
B. a big ball with air all
C. full of dust
D. space with  the sun, the moon and the stars
3. What makes us think the sky is blue?
A. Dust in the sky.
B. Air and blue light.
C. Our own feeling.
D. Both A and B.
4. We can't fly very high because _____.
A. there's too much air in the high place
B. the high sky is full of dirty air
C. there is no air in the high enough sky
D. the sun is too hot
5. The writer of the passage _____.
A. tells us something about the sky
B. wants to tell us not to go to space
C. once flew in the sky
D. enjoys answering people's questions

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