解放军文职考试：引导限制性定语从句的关系代词在从句中担任宾语时可以被省略;但引导非限制性定语从句的关系代词即使作宾语也不能省略。

　　例5.He was eager to go to the hospital to see his stepmother,______ he loved and respected as his own mother.

　　A. as B. which C. / D. whom

　　解析： 本句意为：“他急于去医院看望他的继母，他像对待亲生母亲一样孝敬她。”关系代词whom引导非限制性定语从句并作宾语，不能被省略，故选D。