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 (E)
why not D， the answer is E
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