单选题 What is the sky(天空)? Where is it? How high is it? What lies above the sky? I am sure that you have asked these questions. They are very difficult to answer, aren't they?
If someone asked you: "What color is the sky? " I think that you would answer: "Blue". I am afraid that you would be wrong. The sky has no color. When we see blue, we are looking at blue sunlight. The sunlight is shining on little bits of dust in the air.
Is the sky full of air? I am sure that you have asked this question, too. We know that there is air all around the world. We could not breathe without air. Planes could not fly without air. They need air to lift their wings. Planes cannot fly very high because as they go higher, the air gets thinner. If we go far enough away from the earth, we find there is no air.
Perhaps we can answer some of our questions now. 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!

单选题 Everybody knows how high the sky is.

【正确答案】 B

【答案解析】文章开头提出了好几个问题：天空是什么?它在哪里?它有多高?……第一段最后一句话是：They are very difficult to answer，aren't they?既然这些都是很难回答的问题，当然不可能每个人都知道答案。

单选题 Scientists are able to answer these questions.

【正确答案】 C

【答案解析】研究这个领域的科学家们肯定能回答这个问题，但文中并没有提及。做短文阅读理解，一定要根据短文进行选择，不可妄自猜测。

单选题 There is air all around the universe (宇宙).

【正确答案】 B

【答案解析】从文章第三段，我们得知：...there is air all around the world. 此处的“world”指的是我们生活的地球。第三段最后一句话：If we go far enough away from the earth, we find there is no air. 说明远离地球的地方就没有空气了。

单选题 Planes cannot fly very high because as they go higher, the air gets thicker.

【正确答案】 B

【答案解析】这个句子错在“thicker”这个词；相反，从文章中我们知道“... as they go higher，the air gets thinner.”飞得越高，空气越稀薄。飞机靠空气抬升机翼，空气越来越稀薄，飞机就无法飞行。