【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正确答案A
