Question no.4

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A light bulb is hanging in a room. Outside of the room there are three switches of which only one is connected to the lamp. In the starting situation, all switches are 'off' and the bulb is not lit. If it allowed to check in the room only once to see if the bulb is lit or not( this is not visible from the outside), how can you determine with which of the three switches the light bulb can be switched on?