A spherical iron ball 10 cm in radius is coated with a layer of ice of uniform thickness that melts at a rate of 50 cm3/min. When the thickness of ice is 15 cm, then the rate at which the thickness of ice decreases is
(a) 5/6pi cm/min
(b) 1/54pi cm/min
(c) 1/18pi cm/min
(d) 1/36pi cm/min
