单选题

　　[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