The old man has three sons，and two are doctorsA. of whichB. of whomC. of themD. of that

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A. of which
B. of whom
(答案→)C. of them
D. of that
解析：考查代词。根据句中连词and可知，后面连接并列分句，非考查定语从句，故用代词them，指代three sons。句意：老人又3个儿子，2个是医生。