Everyone wants to reduce pollution. But pollution problem is ________ (1) complicated as it is serious. It is complicated ________ (2) much pollution is caused by things that benefit people. For example, exhaust ________ (3) automobiles causes a large percentage of all air pollution. But automobiles provide transportation for millions of people. Factories discharge much of the material ________ (4) pollutes the air and water, but factories give employment to a large number ________ (5) people. Too much fertilizer or pesticide can ruin soil, but fertilizers and pesticide are important aids to the growing of crops. Thus, to end or reduce pollution immediately, people would have to stop using many things that benefit ________ (6). Most people do not want to do that, of course. But pollution can be gradually reduced in several ways.

Why is the pollution problem complicated?