¡¡¡¡[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