It is desired to increases the heat dissipation rate over the surface of an electronic device of spherical shape of 5 mm radius exposed to convection with h=10 W/m2 K by encasing it in a spherical sheet of conductivity 0.04 W/mK. For maximum heat flow, the diameter of the sheet should be (IES-96)
(a) 18mm (b) 16mm
(c) 12 mm (d) 8 mm