自考《英语（二）》真题练习：完成句子题（8.24）

单选题

[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