单项选择题

Passage Two
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 developed countries in particular, emphasis has shifted 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 occur more frequently in cities whose water supplies have comparatively 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 chemicals 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 significance 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 attractive in appearance but also meet certain other conditions. Sports such as swimming involve direct contact with water, and so water standards are raised higher.
The range of water quality necessary for industrial processes varies greatly with the intended use of water. The water used in steel mills, for example, must have a lower chloride(氯物) 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 contain little iron and carbon dioxide.
In the last paragraph, the author mentions all of the following EXCEPT ______