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 _standing_____ room on the earth 600 years later. That will be a very serious problem.So man is _thinking_____ 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 __is____ no air, no plants, no life there. So scientists are doing experiment in many ways. They have to solve many problems before____ man can live on the moon. I'm sure our dream will _come true _____ in the next century.