Problem: A photon gas is at thermal equilibrium at temperature $T.$ The mean number of photons in an energy

state $=$ is:

$($a$)$ exp$(\frac{}{k_{B}T})+1$

$($b$)$ exp$(\frac{}{k_{B}T})-1$

$($c$)(exp(\frac{}{k_{B}T})+1{)}^{-1}$

$($d$)(exp(\frac{}{k_{B}T})-1{)}^{-1}$