The old professor has three sons， are doctors.
A. all of them
B. all of whom
C. all of that
D. all of which
网友回答
A. all of them
(答案→)B. all of whom
C. all of that
D. all of which
解析：考查定语从句。本题的先行词是three sons，在定语从句中whom指代先行词在句中做of的宾语，因为先行词是人，所以使用whom表示。句意：那个老教授有三个儿子，所有的儿子都是医生。