The time required in seconds to cool the steel ball

Question

A steel ball of diameter 60 mm is initially in thermal equilibrium at 1030⁰C in a furnace. It is suddenly removed from the furnace and cooled in ambient air at 30⁰C, with convective heat transfer coefficient h = 20 W/m²K. The thermo-physical properties of steel are: density $\rho$= 7800 kg/m³, conductivity k = 40 W/mK and specific heat c = 600 J/kgK. The time required in seconds to cool the steel ball in air from 1030⁰C to 430⁰C is              

      (a) 519                                                                (b) 931

      (c) 1195                                                              (d) 2144