13、There are many people in the world now. There will be a lot more people next century. The earth will be very crowded and there will be a little room for each person. Maybe there will only be _ _ _ 21 _ _ _ room on the earth 600 years later. That will be a very serious problem.
So man is _ _ _ 22 _ _ _ of a way to solve the problem.
The moon is the satellite of the earth. Maybe man will go to stay on the moon one day. But there _ _ _ 23 _ _ _ no air, no plants, no life there. So scientists are doing experiment in many ways. They have to solve many problems _ _ _ 24 _ _ _ man can live on the moon. I'm sure our dream will _ _ _ 25 _ _ _ in the next century.