Which of the following most logically completes the passage? Using new detection techniques, researchers have found trace amounts of various medicinal substances in lakes and rivers. Taken in large quantities, these substances could have serious health effects, but they are present in quantities far too low to cause any physiological response in people who drink the water or bathe in it. Nevertheless, medical experts contend that eliminating these trace amounts from the water will have public health benefits, since __________. A. some of the medicinal substances found in lakes and rivers are harmless to humans even if taken in large quantities B. some of the medicinal substances found in lakes and rivers can counteract possible harmful effects of other such substances found there C. people who develop undesirable side effects when being treated with medicines that contain these substances generally have their treatment changed D. most medicinal substances that reach lakes or rivers rapidly break down into harmless substances E. disease-causing bacteria exposed to low concentrations of certain medicinal substances can become resistant to them 通过最新的技术，科学家可以追踪湖底或者河流中的药物成分。数量多的情况下会有很大的健康影响。但是现在来看数量太少不足以对人有什么影响。不过医学家说跟踪这些物质的变化对于人类的健康还是有帮助的，问为什么？ Ｂ选项说这些物质可能会跟在湖里的其他物质发生反应从而影响人体健康。