A closed cubical box is made of a perfectly insulating material walls of thickness `8cm` and the only way for heat to enter or leave the box is through two solid metallic cylindrical plugs, each of cross-sectional area `12cm^(2)` and length `8cm`, fixed in the opposite walls of the box. The outer surface `A` on one plug is maintained at `100^(@)C` while the outer surface `B` of the other plug is maintained at `4^(@)C`. The thermal conductivity of the material of each plug is `0.5 cal//.^(@)C//cm`. A source of energy generating `36 cal//s` is enclosed inside the box. Assuming the temperature to be the same at all points on the inner surface, the equilibrium temperature of the inner surface of the box is
A. `62^(@)C`
B. `46^(@)C`
C. `76^(@)C`
D. `52^(@)C`
