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

He works as a teacher now,but he ___ on a farm for 3 years.A.worked B.has worked C.had worked D.woHe 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.