When permits for the discharge of chemicals into a waterway are issued, they are issued in terms of the number of pounds of each chemical that can be discharged into the waterway per day. These figures, calculated separately for each chemical for which a permit is issued, are based on an estimate of the effect of the dilution of the chemical by the amount of water flowing through the waterway. The waterway is therefore protected against being adversely affected by chemicals discharged under the permits.
The argument depends on the assumption that
A．relatively harmless chemicals do not interact with each other in the water to form harmful compounds.
B．there is a swift flow of water in the waterway that ensures rapid dispersion of chemicals discharged.
C．there are no chemicals for which discharge into waterways is entirely prohibited.
D．those who receive the permits do not always discharge the entire quantity of chemicals that the permits allow.
E．(E) the danger of chemical pollution of waterways is to be evaluated in terms of human health only and not in terms of the health of both human beings and wildlife.
参考答案：A
解析： 因为保护河道的许可证的发放是基于对每种化学物质的最高允许排放量的计算，因此要确保河道内的水在许可证的保护之下，还必须确保那些根据许可证的要求排放到河道内的水中的化学物质相互之间不会发生形成有害物质的反应。由此分析可知(A)为正确答案；而(B)、(C)、(D)和(E)均为无关选项。
