根据短文内容的理解，选择一个最佳答案, 回答问题。
(C)
If someone asks you, "What colour is the sky?" I think the answer will be "blue". I'm afraid that you are wrong. Because the sky has no colour.
What's in the sky? Is the sky full of air? I'm sure that you have asked these questions. We all know that there's air all around the world. We can not live without air. Planes can't fly very high because as they go higher, the air gets thinner. If we go far enough away from the earth, we can find that there is no air. The sky is space（太空）. In this space there is nothing but the sun, the moon and the stars.
People always want to know more about other worlds in space. They have looked at them through （通过） telescopes （望远镜） and in this way they have known much. They know a lot about the moon now. But they want to know more about space. I'm sure they will go farther and farther until they know all about it.
—What's this? —It is a m