单选题

　　1.【完成句子】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.

　　Industrially contaminated sites usually require a thorough clean-up due to __________.

　　A.the persistence of heavy metals

　　B.industrially polluted soils

　　C.naturally polluted soils

　　D.rock and soil chemistry