Heat is generated steadily in a 3-cm-diameter spherical ball. The ball is exposed to ambient air at 26°C with a heat transfer coefficient of 7.5 W/m²·K. The ball is to be covered with a material of thermal conductivity 0.15 W/m · K. The thickness of the covering material that will maximize heat generation within the ball while maintaining ball surface temperature constant is
(a) 0.5 cm

(b) 1.0 cm

(c) 1.5 cm

(d) 2.0 cm

(e) 2.5 cm