閱讀理解：
    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
    正確答案：B
    答案解析：本段文字講的是：受到工業(yè)污染的土壤含有重金屬，而重金屬很穩(wěn)定不容易降解，其中的某些微量重金屬能使人致癌或中毒。可以看出其中心內(nèi)容是：受污染土地的潛在危險，選擇B。