The figure shows a system of two concentric spheres of radii `r_1 and r_2` are kept at temperature `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. `((r_(2)-r_(1)))/((r_(1)r_(2)))`
B. `In((r_(2)))/((r_(1)))`
C. `(r_(1)r_(2))/((r_(2)-r_(1)))`
D. `(r_(2)-r_(1))`