I work with several master's and Ph.D. students, almost all of__

I work with several master's and Ph.D. students, almost all of_____are women, during the course of my research at a large state university.
A、whom
B、which
C、that
D、them
【正确答案】：A
【题目解析】：这里whom作为定语从句的关系代词修饰先行词students。