训练题
一、完形填空
For thousands of millions of years the moon has been going round the earth.At that time，the moon was the __1__ satellite(卫星)of the earth.Today，__2__，the earth has many other satellites.All of them are __3__ by man.These manmade satellites are very much smaller than the moon.However，some of them will still be going __4__ the earth thousands of years from now.
