The rate of fall of the temperature of the ball at the beginning of cooling will be, in K/s

Question

A small copper ball of 5 mm diameter at 500 K is dropped into an oil bath whose temperature is 300 K. The thermal conductivity of copper is 400 W/m. K, its density 9000 kg/m³ and its specific heat 385 J/kg. K. If the heat transfer coefficient is 250 W/m².K and lumped analysis is assumed to be valid, the rate of fall of the temperature of the ball at the beginning of cooling will be, in K/s                                   

           (a) 8.7                                                                  (b) 13.9

            (c) 17.3                                                               (d) 27.7