单选题

　　[概括大意]Heavy metals are persistent； they do not break down to other chemicals in the environment. Industrially polluted sites usually undergo intensive clean-up and rehabilitation because heavy metals are a health concern once they enter the food chain. Some trace metals are alleged to cause cancer and are also known to cause poisoning.

　　The main idea of this paragraph is__________.

　　A.The Nature of Heavy Metals

　　B.Potential Hazards of Human Contaminated Soils

　　C.Heavy Metals and Food Chain

　　D.Research on Channels of Heavy Metals Getting into Human Food Chain