Radiative heat transfer rate between parallel gray surfaces

Question

The radiative heat transfer rate per unit area (W/m²) between two plane parallel gray surfaces (emissivity = 0.9) maintained at 400K and 300K is ($\sigma _{b}$= Stephen Boltzmann constant 5.67 $\times$ 10^–8 W/m²K⁴)                        

      (a) 992                                                                (b) 812

      (c) 464                                                                (d) 567