NOT OPEN / DOOR / UNLESS / YOU / SURE / WHO / VISITOR / BE. NOT...

64. not open / door / unless / you / sure / who / visitor / be. A. Unless you are sure who is the visitor, don't open the door. B. Not open the door unless you are sure who is the visitor.C. Your are not sure who the visitor is, unless you open the door.D. Don't open the door unless you are sure who the visitor is.