What is the sky? How high is it? What color is it? What kinds of birds can live above(在……上) it? These questions are difficult to answer, aren't they?

In fact, the sky has no colors. Then why does the sky look blue?

We know that there is air around the world. When planes fly, they need air to lift(抬起) their wings(翼). Planes can't fly very high. That is because the air gets thinner when they go higher. The same will happen to us if we go high enough. If we go up about 1，200 kilometers from the earth, we'll find no air there. Some birds can fly into the sky as high as a plane, but they cannot fly or live without air.

Looking up above us, you will see the blue sky with white clouds. It is usually light blue. The color will be bluer when you see on the top of a high mountain. And you will see fewer clouds in the sky between you and the sun.

Sometimes the sky doesn't look blue. When the sun rises(上升) or falls, you will see red, orange, and other colors in the sky. Such are the most beautiful moments(時(shí)刻) of the day.

Who can see the colors of the sky high above the earth? The astronauts! They can see those beautiful colors of the sky from outer space. However, before flying into outer space, they sit in a spaceship high above the earth, and there is no air at all. The sky will be that dark and its color will be black.

Now you may understand that we see colors because the air scatters(使散射) the sunlight. Light(光) travels through the air. Without air, there would be no light or colors.

1.Planes can't fly as high as possible because ________.

A.the sky is blue　　　　
 B.the wings are heavier

C.the air gets thinner D.the sky has no colors

2.When you stand on the top of a high mountain, you will see ________.

A.more air B.more colors C.bluer skies D.more clouds

3.We might see many colors in the sky at ________

A.6：00 a．m. B.10：00 a．m. C.1：00 p．m. D.3：00 p．m.

4.The passage mainly talks about ________.

A.why planes can't fly high

B.when we can see more colors of the sky

C.why the sky looks blue

D.why the sky changes(改變) colors

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