Heat flow across a hollow sphere of inner radius 'r1' and outer radius 'r2' is directly proportional to

A. (r2 - r1)/r1. r2

B. r1. r2/(r2 - r1)

C. (r2 + r1)/r1. r2

D. r1. r2/(r2 + r1)

Please do not use chat terms. Example: avoid using "grt" instead of "great".

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