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  3. Municipal sewage is of relative]y recent origin as a pollutant. It was first brought to public attention in the 19th century by

Municipal sewage is of relative]y recent origin as a pollutant. It was first brought to public attention in the 19th century by

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问题      Municipal sewage is of relative]y recent origin as a pollutant. It was first brought to public attention in the 19th century by a London physician who showed that the city’s cholera outbreak had been caused by just one contaminated well. Even though the contamination of drinking water by disease germs has been nearly eliminated in this country, hundreds of communities are still discharging raw sewage into streams and rivers.
     When we consider that this sewage contains effluents from toilets, hospitals, laundries, industrial plants, etc. , then the potential of the pollutants as a health hazard is apparent.
     The problem of municipal sewage disposal is complicated by the fact that, years ago, most cities combined their storm and waste disposal sewers. Many of these combined systems work well, but others cannot cope with sudden heavy rains. When such storms occur, water mixed with sewage may flood and disable treatment plants unless bypassed, untreated, into a stream. In either case; the people may have little protection for several days from these wastes that may contain disease germs.
     Even if adequately treated to eliminate the health hazard, sewage is aesthetically undesirable because of odors and colors produced. Detergents have posed a particular disposal problem. Although there is no indication that they are injurious to health, they can cause foaming, which can clog treatment plants and, at the least, spoil the scenic beauty of streams.
     Rural and suburban residents should be aware that septic tanks and cesspools are a potential source of pollution to ground water supplies. This is especially true in the suburban areas with a high population density and with
no municipal sewage disposal and treatment system available. In some areas, sewage disposal is accomplished by cesspools. Soil research is furnishing guidelines for more effective and safer use of systems such as these.
In developing the main point, the author makes use of ______.
选项 A、scientific arguments
B、convincing testimony
C、common sense observations
D、analogy
答案C
解析 此题为篇章结构题。作者首先提出排污系统为污染源，然后通过观察、分析、归纳来阐述观点，而没用A项“科学论证”、B项“令人信服的证据”或D项“类比”。
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