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The figure shows a system of two concentric spheres of radii r1 and r2 and kept at temperatures T1 and T2, respectively. The radial rate of flow of heat in a substance between the two concentric spheres is proportional to

(a) r1r2r1-r2
(b) r1-r2
(c) r1-r2r1r2
(d) In r2r1