题目内容：
Questions are based on the following passage.Water pollution affects man's health, recreational use of water, industrial of water and sense of beauty. The spread of water-carried disease is of great concern. In highly _47_ countries in particular, emphasis has _48_ from concern over bacterial
disease to concern over water-carried viral (病毒性的) disease. Viral hepatitis (肝炎), for example, has been found to 49 more frequently in cities whose water supplies have 50 high levels of water impurity. Poisonous chemicals found in streams that are ultimately used for water supplies also create an important public health problem.
It is clear that poisonous 51 must be excluded from water, but more and more chemicals from new pesticides,fertilizers, and other new products have been found in water. Since the public health 52 of these chemicals is not yet fully known, they are of increasing concern. The chemicals from fertilizers are also finding their way into water supplies from groundwater pollution.
As man's desire for water-based recreation increases, greater demands are placed on the quality of water. The water must not only be 53 in appearance but also meet certain other conditions. Sports such as swimming involve _54__ contact with water, and so water standards are raised higher.
The range of water quality necessary for 55 processes varies greatly with the intended use of water. The water used in steel mills, for example, must have a lower chloride (氯化物 ) level than that accepted for drinking water. Cooling water can Of ten be of comparatively low sanitary quality. A large part of the water used in the paper-making industry can be of relatively low quality in some aspects, but must 56 little iron and carbon dioxide.
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请作答（47）______
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