自考《英语(二)》试题练习:完成句子(2.1)
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