【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
【题目解析】：本段提到：受到污染的土壤含有重金属，而且重金属很稳定，不容易降解，所以要对重金属进行彻底的清除。答案选择A。
