Task: Question Answering
Input (English): Problem: there are 56 lights which are functional and each is controlled by a separate on / off switch . two children x and y start playing with the switches . x starts by pressing every third switch till he reaches the end . y , thereafter , presses every fifth switch till he too reaches the end . if all switches were in off position at the beggining , how many lights are switched on by the end of this operation ? 
Options: a ) 18 , b ) 19 , c ) 17 , d ) 15 , e ) 16
Output (English): c