The figure shows a system of two concentric spheres of radii \( r_{1} \) and \( r_{2} \) are kep...

The figure shows a system of two concentric spheres of radii \( r_{1} \) and \( r_{2} \) are kept at temperatures \( T_{1} \) and \( T_{2} \), respectively. The radial rate of flow of heat in a substance between the two concentric spheres is proportional to
(a) \( \operatorname{In}\left(\frac{r_{2}}{r_{1}}\right) \)
(b) \( \frac{\left(r_{2}-r_{1}\right)}{\left(r_{1} r_{2}\right)} \)
(c) \( \left(r_{2}-r_{1}\right) \)
(d) \( \frac{r_{1} r_{2}}{\left(r_{2}-r_{1}\right)} \)