根据括号里的单词提示，完成短文填空。
  The Earth is our home and we must take good  (careful) of it. That is to say we must keep the land, air and water clean. Many years ago, pollution  (be) not so serious  the population of the Earth was not so  (large) as today. There was enough fresh air, good agricultural land and clean water. When the land or the river was dirty and  (pollute) in one place, men left there and moved to another place. People are polluting  whole world now. In some aspects, it seems that the more people there are in one place, the  (bad) environment they may live in. Perhaps you are worried that it is endless for people  (produce) pollution.  (Lucky), more and more people have come to know the danger of pollution.  And we can find more and more people are  (work) hard to protect our Earth.