Toughest Popular Puzzle

Toughest Popular Puzzle / Question
(Source : http://hardest-puzzle.blogspot.com/2...-question.html
)

Outside a room there are three light switches. One of switch is
connected to a light bulb inside the room.
Each of the three switches can be either 'ON' or 'OFF'.

You are allowed to set each switch the way you want it and then enter
the room(note: you can enter the room only once)

Your task is to then determine which switch controls the bulb ??


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