All other doors were looked,including the door to the reading room where I ___something useful.
问题描述：
All other doors were looked,including the door to the reading room where I ___something useful.
A.could have found B.could find
C.would D.must have found
答
应该是all the doors were locked吧.选B,排除法,could have done表示原本可以做某事,但结果没做成.must have done是命令语气,表示某人必须做某事.C根本没动词所以不选.B用could是因为前面用的也是过去式.