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[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

答案解析：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。