Mr. Smith, who often helped passers-by repair their bikes, said that what he did was not p

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A. relevant
B. optional
C. suitable
(答案→)D. voluntary
解析：考查形容词辨析。句意：史密斯先生，经常帮助行人修理自行车，他说他做的事情不要报酬，是自愿的。A相关的；B可选择的；C合适的；D志愿的，无偿的。not...but...不是......，而是......，故选D。