Mr. Duncan raised an unnecessary question he then failed to find an answer.A. to whichB.

问题补充：

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

答案：

(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。