People all over the world today are beginning to hear and learn more and more about the problem of pollution. (14) Pollution is caused either by man's release of completely new and often artificial (人造) substances into the environment, or by releasing greatly increased amounts of a natural substance (物质), such as oil from oil tankers into the sea. Whatever its underlying reasons are, there is no doubt that much of the pollution caused could be controlled if only companies, individuals and governments would make more efforts. In the home there is an obvious need to control litter and waste. Food comes wrapped up three or four times in packages that all have to be disposed of; drinks are increasingly sold in bottles or tins which cannot be reused. This not only causes a litter problem, but also is a great waste of resources, in terms of glass, metal and paper. Advertising has helped this process by persuading many of us not only to buy thing we neither want nor need, but also to throw away much of what we do buy. Pollution and waste combine to be a problem everyone can help to solve by cutting out unnecessary buying, excess use and careless disposal (处理) of the products we use in our daily lives. The main cause of pollution is______.