Mr white_______in this school for20 years before he moved to LondonA has taught B taught C had taught 解释为什么选这个选项

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Mrwhite_______inthisschoolfor20yearsbeforehemovedtoLondonAhastaughtBtaughtChadtaught解释为什么选这个选项Mrwhit

Mr white_______in this school for20 years before he moved to LondonA has taught B taught C had taught 解释为什么选这个选项
Mr white_______in this school for20 years before he moved to London
A has taught B taught C had taught
解释为什么选这个选项

Mr white_______in this school for20 years before he moved to LondonA has taught B taught C had taught 解释为什么选这个选项
C
后面的 before he moved to London为过去时,而Mr white_______in this school for20 years 发生在后面的 before he moved to London前,故用过去完成

C
从句中，moved to London发生在过去，由before可见主句动作发生在过去的过去，而过去的过去就要用过去完成时

C，过去完成时表示过去的过去，在过去的某一个动作之前发生的动作。这里before he moved to London已经说明了teach是在move之前发生的动作，move已经是过去式了，teach就是过去的过去，需要用过去完成时

Mr white_______in this school for20 years before he moved to LondonA has taught B taught C had taught 解释为什么选这个选项 Mr green will go to Beijing 什么 7:00 什么 October 11th ,2011 Mr mr mr. Mr. Mr. mr. Mr MR Mr Mr. mr. Mr mr. Mr. MR, MR