Task: Question Answering
Input (English): Problem: a motorcyclist x drives along a circular fence at a rate of 2 rounds per hour and another motor cyclist y at a rate of 4 rounds per hour . after how long they will cross each other for the first time ? 
Options: a ) 10 min , b ) 20 min , c ) 30 min , d ) 40 min , e ) none of these
Output (English): c