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  Who, whom和whose可用于非限制性定语从句中,代表某人或某人的,此时不能用that。如: 
          My sister, who is a nurse, came home for a few days. 
          我姐姐回家住了几天,她是个护士。 
          which 
          which代表物,在从句中可作主语或宾语,作宾语时可以省略,但作介词宾语且介词位于其前时不能省略。例如: 
          The book which helps you most are those which make you think most. 
          最能使你获益的书是那些最能让你深思的书。 
          This is a factor which we must not neglect. 
          这是一个我们绝不能忽略的因素。 
          Where is the book from which you quoted this sentence? 
          你引用这句话的那本书在哪儿? 
          which可用于非限制性定语从句中,代表物。如: 
          The book, which I bought second-hand, is made of oak. |   
 
 
 
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