As a result of pollution, Lake Erie, on the borders of the USA and Canada, is now without many living things. Pollution in water is not simply a matter of "poisons" killing large numbers of fish overnight. Very often the effects of pollution are not noticed for many months or years because the first organisms to be affected are either plants or plankton . These organisms are the food of fish, birds, and other creatures. When this food disappears, the fish and birds die too. In this way a whole food chain can be wiped out, and it is not until dead fish and water birds are seen at the river's edge or on the sea shore that people realize what is happening. Where do the substances which pollute water come from? There are two main sources, sewage and industrial waste. As more detergent is used in homes, more of it is finally put into our rivers, lakes and seas. Detergents harm water birds by breaking down the natural substances which keep their feathers _ Sewage itself, if not properly treated, makes the water dirty and prevents all forms of life in rivers and the sea from receiving the oxygen they need. Industrial waste is even more harmful since there are many highly poisonous materials in it, such as copper and lead. So if we want to stop this pollution, the answer is simple. Sewage and industrial waste must be made clean before flowing into the water reservoirs. It may already be too late to save some rivers and lakes, but others can still be saved if the correct action is taken at once. Living things die in a lake or river because there is no _ . A) water B) fish C) Poison D) oxygen
D