The earth is not the only【C1】______that travels around the sun. With it are eight other pl

anets, fellow members of the sun's family.

Two of them are【C2】______while the other six【C3】______from the sun than the earth is. The farther they are, the longer【C4】______they【C5】______around the sun. People noticed long ago that these traveling bodies moved around in the sky【C6】______definite paths. It is a【C7】______called gravity that【C8】______them in their paths.

We know that every little bit of matter in the【C9】______pulls upon every other bit of mater. The pull between two bodies is【C10】______to the product of their masses. Because the sun is so large that pulls【C11】______the sun and the planets are thus great. If it【C12】______not for these pulls, the planets【C13】______off into space. In the same way【C14】______exists a pull between the earth and the moon, which keeps the moon【C15】______in its orbit around our planet, the earth. The gravity holds you to its surface, and pulls back to it the ball which you【C16】______into the air. Of course the ball also pulls【C17】______the larger earth, but the earth is so much larger that the pull【C18】______.

Now remember that large bodies exert a greater pull on any body than smaller ones which contain【C19】______material. But each object in the universe, no matter how small, pulls on all other objects【C20】______extent.

【C1】

A．thing

B．body

C．matter

D．substance