Radiation Heat Transfer & Emissivity

For diffuse, gray radiation exchange between two surfaces 1 and 2 of areas $A_1$ and $A_2$, the reciprocity theorem states:

Options:

• (A) $A_1 F_{12} = A_2 F_{21}$
• (B) $F_{12} + F_{21} = 1$
• (C) $A_1 F_{21} = A_2 F_{12}$
• (D) $F_{12} = F_{21}$ for all geometry