单选题

　　[概括大意]So far there is no data to suggest that people living on metal-rich soils experience a potential health hazard. The levels of metals within naturally contaminated soils are generally not high enough to cause serious health problems. Living on metal-rich soils does not represent a health risk unless large quantities of soil are digested or metal-rich dust is inhaled. However， small children are particularly exposed to metal-rich topsoil in playgrounds and gardens. They are also the most likely ones to eat potentially dangerous metal-rich soil.

　　The main idea of this paragraph is__________.

　　A.Potential Hazards of Human Contaminated Soils

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

　　C.Long- term Health Effects on Health Problems

　　D.No Evidence to Indicate Bad Effects of Naturally Contaminated Soil