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阅读理解：

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.

The main idea of this paragraph is__________.

A.The Nature of Heavy Metals

B.Potential Hazards of Human Contaminated Soils

C.Heavy Metals and Food Chain

D.Research on Channels of Heavy Metals Getting into Human Food Chain

正确答案：B

答案解析：本段文字讲的是：受到工业污染的土壤含有重金属，而重金属很稳定不容易降解，其中的某些微量重金属能使人致癌或中毒。可以看出其中心内容是：受污染土地的潜在危险，选择B。

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