Mr. Duncan raised an unnecessary question he then failed to find an answer.
A. to which
B. for which
C. with which
D. by which
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(答案→)A. to which
B. for which
C. with which
D. by which
解析：句意：邓肯先生提出了一个没有必要的问题，他后来也没有找到问题的答案。这里是定语从句，先行词是question，an answer to an unnecessary question问题的答案，这里需用介词to，which在这里作介词的宾语，故选A。