填空题. A．balanced B．developed C．affect D．contain E．consistently F．creatures G．shifted H．agricultural I．chemicals J．occur K．attractive L．significance M．direct N．comparatively O．industrial Water pollution affects man's health, recreational use of water, industrial use of water and sense of beauty. The spread of water-carried disease is of great concern. In highly 21 countries in particular, emphasis has 22 from concern over bacterial disease to concern over water-carried viral disease. Viral hepatitis, for example, has been found to 23 more frequently in cities whose water supplies have 24 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 25 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 26 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 27 in appearance but also meet certain other conditions. Sports such as swimming involve 28 contact with water, and so water standards are raised higher. The range of water quality necessary for 29 processes varies greatly with the intended use of water. The water used in steel mills, for example, must have a lower chloride (氧化物) level than that accepted for drinking water. Cooling water can often 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 30 little iron and carbon dioxide.