[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