A boy is cycling such that the wheels of the cycle are revolving at the rate of $140$ revolutions per minute. If the diameter of the wheel is $60\;cm$ then the speed per hour with which the boy is cycling is _________ km/hr

Question

$(A) \; 20 \;km/hr \\(B)\;16\;km/hr \\(C)\;15 \;km/hr \\(D)\;12\;km/hr $