[音频]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 ground. When land was used up or water became (1)______ , man moved on to another place. At first it was all right because there was plenty of clean air, land and (2)_______ . And there were (3)_______ people. But the growing population and modern industry have changed the situation. Today, pollution has brought serious (4)______ to our health and life.