The thermal resistance in the wall of the tube is

Question

Consider a long cylindrical tube of inner and outer radii, r_i and r_o, respectively, length, L and thermal conductivity, k. Its inner and outer surfaces are maintained at T_i > T_o). Assuming one dimensional steady that heat conduction in the radial direction, the thermal resistance in the wall of the tube is                                  

      (a) $\frac{1}{2\pi L}\imath n\left ( \frac{r_{i}}{r_{o}} \right )$                                                   (b)$\frac{1}{2\pi r_{i} L}$

      (c) $\frac{1}{2\pi L}\imath n\left ( \frac{r_{o}}{r_{i}} \right )$                                                    (d) $\frac{1}{4\pi L}\imath n\left ( \frac{r_{o}}{r_{i}} \right )$