Mary received training in business English for three months, she found a job in a big fore

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A. after that
(答案→)B. after which
C. after it
D. after when
解析： 句意为：玛丽接受了商务英语培训，长达3个月，之后再一家外企找到了一份工作。
从主句和从句的关系看，从句不缺成分，则由关系副词充当，关系副词=相应的介词+which/whom。据句意，在此之后，选B。因为that，it不做非限制性定语从句的关系词。
考点:定语从句关系词的考察