The old man has four sons, and two turned doctorsA. of whichB. of whomC. of themD. of th

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A. of which
B. of whom
(答案→)C. of them
D. of that
解析：考查定语从句。句中有逗号，不小心会误以为是非限制性定语从句，但and连接前后两分句，故不是定语从句。“其中的两个”是two of them。故选C。