What is the sky(天空)? Where is it? How high is it? What lies above it? These questions are hard 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(翼) . Planes cannot fly very high because when they go high, the air gets thinner. If we go up about 1,200 kilometers from the earth, we find there is no air. Perhaps(也许) we can answer some of our questions now. The sky is space(空间). In this space there is only the sun, the moon and all the stars. 6.What is the sky? A. It’s the clouds       B. It’s space.     C It’s air      D It’s nothing 7. The sky _____ A. is blue        B. is white        C. is black     D. has no color 8. Planes cannot fly very high because _________. A. the air becomes thinner                         B. the flying machine is not good C. people will be afraid if the planes fly too high      D. it is not necessary for planes to fly so high 9 If we go up about 1,200 kilometers from the earth, we find ___________. A. there is more air   B. there is less air   C .there is nothing but air   D there is no air 10. In space there is _________. A. nothing       B. air everywhere       C. everything       D. only the sun , the moon and all the stars