What is the expression for the thermal conduction resistance to heat transfer through a hollow sphere of inner radius r1 and outer radius r2 , and thermal conductivity K? (IES-07)
(a)$\tfrac{r_2-r_1*(r_1r_2)}{4\pi k }$
(b)$\tfrac{(r_2-r_1)4\pi k}{r_1*r_2}$
(c)$\frac{r_2-r_1}{4\pi k(r_1*r_2)}$
(d) None of these<!--[if gte msEquation 12]>None nof these<![endif]-->