定于从句问题① He has three sons, none of __A___ is a doctor. ② He has three sons, but none of ___B_ is a doctor. ③ He

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定于从句问题
① He has three sons, none of __A___ is a doctor. ② He has three sons, but none of ___B_ is a doctor. ③ He has three sons, ____D___ are doctors.
④ He has three sons; _C___ are
doctors.
A. whom B. them C. they D. who
告诉我为什么这样填?

介词+whom,them,他们是宾格形式
they,是主格,充当主语