They need air to lift their wings.Airplanes 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 other stars.-|||-Let`s say something about the moon,which is about 384,000 kilometers away from the-|||-earth.An airplane cannot fly to the moon but a rocket can.The rocket is not made of rubber but-|||-strong and light metal.There is gas inside the rocket. The gas is made very hot.When it rushes-|||-out of the end of the rocket,the rocket is pushed up into the air.-|||-Rockets with men inside them have already reached the moon. One day the rockets may be-|||-able to go anywhere in space.-|||-() 41.What is the sky?-|||-A.Heaven. B.Air. C.Space. D.The world.-|||-() 42.What color is the sky?-|||-A.White. B.Blue. C.Gray. D.Colorless.-|||-() 43.Airplanes can`t fly very high because __-|||-A.the air gets thicker B.the air gets thinner-|||-C.they have no wings D.they needn`t air-|||-() 44.Which of the following statements is true?-|||-A.Only rockets can fly to the moon.-|||-B.Neither airplanes nor rockets can fly to the moon.-|||-C.Both airplanes and rockets can fly to the moon.-|||-D.Only airplanes can fly to the moon.-|||-() 45.The rocket is made of __-|||-A.rubber B.heavy metal C.light and strong metal D.hot gas