A Bit of Brain Work

We have a room with a bulb fixed to its ceiling. Outside the room we have three switches A,B,C.
One of them is for the bulb inside the room
what we are supposed to do is to find that which one out of the three switches is the one for the bulb inside the room?
simple is'nt it ? just keep lookin whether the bulb glowes with A turned on ,or B or C.

But the problem comes with some constraints:-
you are allowed to go inside the room only once (its locked from outside). And you just have to use your brain i.e. without any equipment.