The doctor she sent her friend is very well known.A. by whomB. at whomC. whomD. to whom

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A. by whom
B. at whom
C. whom
(答案→)D. to whom
解析：考查定语从句中的介词搭配。本句中的to是动词短语send her friend to…中的介词。句意：她送她的朋友去的那个医生是很著名的。故D正确。