Task: Question Answering
Input (English): Problem: there are 172 lights which are functional and each is controlled by a separate on / off switch . two children a and b start playing with the switches . a starts by pressing every third switch till he reaches the end . b , 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 ) 83 , b ) 85 , c ) 87 , d ) 79 , e ) 89
Output (English): d