An infinite parallel planes with emissivities e₁ and e₂, the interchange factor for radiation from surface 1 to surface 2 is given by

A. (e₁ + e₂)/ e₁ + e₂ - e₁e₂

B. 1/e₁ + 1/e₂

C. e₁ + e₂

D. e₁e₂

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