Have you been to Shanghai?Yes,I ( )therefor 3 days last mont

Have you been to Shanghai?Yes,I ( )therefor 3 days last month.答案是havebeen用 have been还是was
网上有的人说用havebeen有的人说用was,究竟哪个?如果用havebeen,后面不是有last month吗?现在完成时态不是不能和last month连用的吗?如果用was,那和前面的问题不是不对应了吗?