Q:

A problem on emissivity/heat transfer?

Can anyone please explain why the answer is D, not C?
I understand that it uses Stefan-Boltzmann equation and the temperature difference, 
but why would you multiply the emissivity of X, e, also to the power of the blackbody (the wall)? 
Thank you in advance.

A:

If I remember correctly, the emissivity is a two-way parameter, and describes the efficiency of transforming heat into radiation and vice versa. It not only determines how efficient radiation is put out, but also determines the efficiency of incoming radiation. Therefore, the radiation by the black-body to body X is also influenced by the emissivity of body X.

