He works as a teacher now,but he ___ on a farm for 3 years.A

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He works as a teacher now,but he ___ on a farm for 3 years.A.worked B.has worked C.had worked D.wo
He works as a teacher now,but he ___ on a farm for 3 years.
A.worked B.has worked C.had worked D.wokes
答案给的是A,但是为什么不能选C过去完成时呢?他现在的工作是作为一名老师,但是他曾经在农场干了三年.
还有我想知道为什么farm前面的介词是on,shcool、hospital等前面都是in啊.

A一般过去时表示整个动作都发生在过去.
没有过去的某一时间作为参照,所以不用过去完成时.
关于介词的使用,通常都是习惯或固定用法,这里也可把农场理解为一片平坦开阔的地方,on表示在其上.
希望有帮助!
再问：是不是只有过去的一个时间点做参照才能用过去完成时？就像将题中的for 3 years改成in 2010是不是就可以选C了？
再答： in 2010𣎴能与过去完成时连用。by⋯可以

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