Here at Red Gate we have installed an extra security measure to help prevent unauthorised access to our Oracle server room. Our server room has no windows, the air vents on the wall are for the air conditioning units and the room (or any light in the room) cannot be seen through them, and the door is solid steel. A 21 digit code unlocks the door, but the lights (bulbs hanging on cords from the ceiling), and monitors can only be turned on by a voice command once you are inside the room. The command is either “A”, “B”, or “C”, and our DBA changes it daily without letting anyone know.

There are three switches marked A, B and C on the wall outside the door, and one of these also works the lights but only when the door is locked shut. You are allowed to touch any or all switches as often as you like before you open the door and enter the room, but once inside, you must say “A”, “B”, or “C”.

You get one chance before the server room door locks and the alarm goes off. How do you know whether A, B or C is the right command? Please post your answers below.

Note: You obviously know the code to get into the room!