enough, health, later,  pass, slowly, ,  population , fast ,  number,  mean,  much

 
Ten thousand years ago, the world’s   was very small. For several thousand years it grew quite  . But during the last three or four hundred years it has grown very . Two thousan years ago , there were only 250 million people on the earth. One thousand six hundred years   , the   was over 500 million . In 1990, there were over five bilion. A UN report said that the word’s population would    six billion by the end of this century. This is just ten years after it reaches five billion. This   we must grow    food, and have smaller families with fewer, butchildren. If we don’t do these things, the results will be bad. There will not be      space (空间) even to stand on the earth.