【单选题】

As a result of pollution, LakeErie, on the borders of the U. S.

A、andCanada, now does not have 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 until many months or years later because the first organisms (生物) to be affected are either plants or planktons (浮游生物). These organisms are the food of fish, birds, and other creatures. When this food disappears, the fish and birds would die too. In this way a whole food chain can be wiped out, and it is not until dead fish and water birds are found at the river’s edge or on the sea shore that people realize what is happening.
Where do the substances that pollute the water come from There are two main sources, sewage (生活污水) and industrial waste.As more detergent (洗涤剂) is used in the homes, more of it is finally put into our rivers, lakes and seas.Detergent harms water birds by breaking down the natural substances which keep their feathers waterproof. 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 more 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 rivers and seas. It may be already too late to save some rivers and lakes, but others can be saved if the correct action is taken at once.
Pollution in water is noticed ______.
[A] when poisons are poured into water
[B] when a great many fish and birds die
[C] when the first organisms are affected
[D] as soon as the balance of nature is destroyed