Two plane coaxial disks are separated by a distance 0.6 m. The lower disk is solid with a diameter 0.80 m and a temperature 300 K. The upper disk , at temperature 900 K, has the same outer diameter but is ring-shaped with an inner diameter 0.40 m. Assuming the disks to be blackbodies, calculate the net rate of radiative heat exchange from disk 1 to disk 2, in kW.