(D)

　　When you run downhill, gravity(亙力) pulls you.  When you throw a ball up, gravity pulls it down. When you sit, gravity holds you down. When you lie down, gravity holds you to the bed.

　　We know gravity is everywhere even though we can't see it. We know it pulls on every stone and every drop of water. It pulls on everything.

　　The gravity of the earth pulls everything towards the center of the earth. You know this when you try to lift a heavy stone. Gravity pulls it down. The more the stone weighs, the more gravity pulls on it. To lift the stone, you must pull up harder than gravity pulls down.

　　The earth has gravity and the moon also has gravity. The moon has less gravity than the earth has. This means the moon's gravity does not pull as hard as the earth's gravity.

　　Do you weigh 60 kilograms? That means the pull of the earth's gravity on you is 60 kilograms. If you were on the moon, you would weigh only 10 kilograms. The pull of the moon's gravity on you would be only 10 kilograms.

　　(     )46. When a person sits on a chair, ________ holds him down.

　　A. weight    B. height    C. gravity    D. magic

　　(     )47. Gravity ________ .

　　A. pulls on everything high up   B. is everywhere

　　C. can be seen everywhere     D. pushes on everything

　　(     )48. You can't lift up a big stone because ________.

　　A. more gravity pulls it down         B. you don't know how to do it

　　C. you are not strong          D. gravity is out of the stone

　　(     )49. If you weigh 50 kilograms, that means ________.

　　A. you can lift up 50 kilograms of things

　　B. the earth's gravity on you is less than 50 kilograms

　　C. you can lift up more than 50 kilograms of things

　　D. the earth's gravity on you is 50 kilograms

　　(     )50. If a stone weighs four kilograms on the moon, it weighs ________.

　　A. 10       B. 24        C. 40        D. 60