C.people who develop undesirable side effects when being treated with medicinesthat contain these substances generally have their treatment changed这句话你理解得不太准确，
它的意思是 当出现了副作用的人们用含有这些物质的药物治疗的时候，他们大体都换了治疗方式。
句子主体是 people generally have their treatment changed
而不是becoz the substances in the medicine used in treatment bring about undesirable side effects so that people have to change their treatment.
这样他就是一个irrelevant选项。
搜了一下发现没人跟我犯同样错误。。。
95. (28538-!-item-!-188;#058&002980)Whichof the following most logically completes the passage?Usingnew detection techniques, researchers have found trace amounts ofvarious medicinal substances in lakes and rivers. Taken in large quantities, these substancescould have serious health effects, but they are present in quantities far toolow to cause any physiological response in people who drink the water or bathein it. Nevertheless, medical expertscontend that eliminating these trace amounts from the water will have publichealth benefits, since __________.A.some of the medicinal substances found in lakes and rivers are harmless tohumans even if taken in large quantitiesB.some of the medicinal substances found in lakes and rivers can counteractpossible harmful effects of other such substances found thereC.people who develop undesirable side effects when being treated with medicinesthat contain these substances generally have their treatment changedD.most medicinal substances that reach lakes or rivers rapidly break down intoharmless substancesE.disease-causing bacteria exposed to low concentrations of certain medicinalsubstances can become resistant to them这道题E的选项很好理解，但是我觉得C也是对的。C说的是服药的人如果碰到了这些物质可能会改变服的药的性质从而影响治疗。所以这个跟E一样都是加强啊~-- by 会员 ran2010zx (2011/9/14 8:43:58)
-- by 会员 houhaibo528 (2011/9/14 13:04:12)
对哦。。。~谢谢哈哈~~~