710. Heat flow across a hollow sphere of inner radius ‘r₁’ and outer radius ‘r₂’ is directly proportional to September 6, 2025 by Admin A. (r₂ – r₁)/(r₁ – r₂) B. (r₂ . r₁)/(r₂ – r₁) C. (r₁ + r₂)/(r₁ . r₂) D. (r₁ . r₂)/(r₁ + r₂) ✅ The correct answer is option B.