For infinite parallel planes with emissivities $$\epsilon_1 and \epsilon_2,$$ the interchange factor for radiation from surface 1 to surface 2 is given by

A

$$\frac {\epsilon_1 \epsilon_2}{\epsilon_1 + \epsilon_2 - \epsilon_1 \epsilon_2}$$

B

$$\frac{1}{\epsilon_1} + \frac {1}{\epsilon_2}$$

C

$$\epsilon_1 + \epsilon_2$$

D

$$\epsilon_1 \epsilon_2$$