阅读理解。 What is the sky? Where is it? How bright is it? What lies above it? These questions are difficult to answer, aren't they? Is the sky blue? The sky has no color. We know that there is air around the world. When planes fly, they need air to lift their wings. Planes can't fly very high because the air gets thinner when they go up higher. If we go up about 1,200 kilometers from the earth, we'll find no air there. When we talk about the sky, we usually mean the atmosphere around the earth. It is like a very thick 'cover' over the earth and keeps it warm. Without the 'cover', the earth would be a very cold place and nothing could live on it. Looking up above us, you will see the blue sky with white clouds. It is usually light blue, but the sky would be dark blue if you go to the top of the highest mountains. And you will see fewer clouds between you and the sun. And if you sit in a spaceship high above the earth, there is no air at all. The sky would be so dark that its color would be black. Sometimes the sky doesn't look blue. When the sun rises and falls, you will see red, orange, and other colors in the sky. These are the most beautiful moments of the day. Now you may understand that we see colors because the air scatters (散射) the sunlight. Light travels through the air. Without air, there would be no light and no colors. 1. Planes can't fly very high because . A. there is air around the world B. the air gets thinner when they go up higher C. it's warmer in the sky D. the air gets thicker when they go up higher 2. What's the meaning of the underlined word 'atmosphere?' A. 氧气 B. 大气层 C. 蓝天 D. 气氛 3. We can see colors of the sky because . A. the sun rises and falls every day B. the sky has lots of colors C. the air scatters the sunlight D. the sunlight has many colors 4. Which of the following is TRUE? A. There is little air without light and colors in the sky. B. The sky will look red, orange and other colors at noon. C. The sky has only one color when the sun rises and falls. D. The sky is dark blue if you go to the top of the highest mountains.