根据首字母提示完成短文

    Ten thousand years ago, the world's p was very small. For several t years it grew quite s But during the last three or four hundred years it g very quickly. Today, it is still growing faster than ever b In 600 years, there will be only standing r on the earth. Each person will have one half to one square metre of space to l in. This means we must grow more food, and have smaller families with fewer, but h children. If we don't do these things, the results will be bad. There will not be e space even to stand in on the e .