阅读理解 这道题选A还是B 原因?

阅读理解 这道题选A还是B 原因?What is the sky?Where is it?How high is?What is above it?These questions are hard to answer,aren’t they? Is the sky blue? The sky has no colour.We know that there is air around the world.When planes fly,they need air to lift their wings(翅膀),planes can’t fly very high,because when they go higher,the air gets thinner.If we go up about1,200 kilometres from the earth,we can find there is no air.Maybe we can answer some of the questions now.The sky is space(空间).In the space there is only the sun,the moon.,and the stars.When planes fly too high,they go very------------. A.hard B.slowly C.fast D.earily