To pollute means to make things unfit or unclean for use. Man has been polluting the earth from the time he first made fire, washed his clothes in the river and threw his waste on the ______. When land was used up or water became dirty, men moved on to another place. At first the problem was not so serious because there was ______ of clean air, land and water, and there were fewer people. All the dirty things could be absorbed by nature. But this is no longer _______. The growth of population and the development of industry have changed that. Nowadays, pollution is beginning to threaten our ______ and life.