根据下列内容，回答196-200题。
Water problems in the future will become more intense and more complex. Our increasingpopulation will tremendously increase urban wastes, primarily sewage. On the other hand, increas-ing demands for water will decrease substantially the amount of water available for diluting wastes.
Rapidly expanding industries which involve more and more complex chemical processes will pro-duce large volumes of liquid wastes, and many of these will contain chemicals which are poi-sonous. To feed our rapidly expanding population, agriculture will have to be intensified. This willinvolve ever-increasing quantities of agriculture chemicals. From this, it is apparent that drasticsteps must be taken immediately to develop corrective measures for the pollution problem.
There are two ways by which this pollution problem can be lessened. The first relates to thetreatment of wastes to decrease their pollution hazard. This involves the processing of solid wastesprior to disposal and the treatment of liquid wastes, or effluents, to permit the reuse of the wateror best reduce pollution upon final disposal.
A second approach is to develop an economic use for all or a part of the wastes. Farm manureis spread in fields as a nutrient or organic supplement. Effluents from sewage disposal plants areused in some areas both for irrigation and for the nutrients contained. Effluents from other process-ing plants may also be used as a supplemental source of water. Many industries, such as meat andpoultry processing plants,are currently converting former waste production into marketablebyproducts. Other industries have potential economic uses for their waste products.
The purpose of this passage is___________.

A.to warn the reader of the dwindling water supply
B.to explain industrial uses of water
C.to acquaint the reader with water pollution problems
D.to demonstrate various measures to solve the pollution problem