The figure shows a system of two concentric spheres of radii $r_1$ and $r_2$ and kept at temperature $T_1$ and $T_2$, respectively. The radical rate of flow of heat in a substance between the two concentric spheres, is proportional to : <br>

Question

( A ) $(r_2 - r_1)$
( B ) $\frac{r_1r_2}{(r_2 - r_1)}$
( C ) $\frac{(r_2 - r_1)}{r_1r_2}$
( D ) $In (\frac{r_2}{r_1})$