The thermal resistance of the cylinder for radial conduction is

Question

A hollow cylinder has length L, inner radius r₁, outer radius r₂, and thermal conductivity k. The thermal resistance of the cylinder for radial conduction is                                                                                                     

      (a) $\frac{In\left ( r_{2}/r_{1} \right )}{2\pi kL}$                                                        (b) $\frac{In\left ( r_{1}/r_{2} \right )}{2\pi kL}$ 

      (c) $\frac{2\pi Kl}{In\left ( r_{2}/r_{1} \right )}$                                                          (d)  $\frac{2\pi Kl}{In\left ( r_{1}/r_{2} \right )}$