高考题!过去完成时与过去式~

高考题!过去完成时与过去式~
有具体时间和地点时用过去式,可是第二题中有“for three years”,看到"for+时间"都是用的完成时,可是这也是具体时间,怎么区分?
题一：--Have you been to the United States?
--Yes,only once.I ___ there for seven years.
A.have stayed B.was staying C.stayed D.had stayed
题二：Susan ____ for three years to be a computer analyst but found her progress discouraging.
A.has worked B.worked C.had worked D.was working