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補(bǔ)全短文

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

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