Consider a spherical shell of radius R at temperature T. The black body radiation inside it can be considered as an ideal gas of photons with internal energy per unit volume $u= \large\frac{U}{V}$$ \infty T^4$ and pressure $P= \large\frac{1}{3}\bigg( \frac{U}{V}\bigg).$ If the shell now undergoes an a diabatic axpansion the relation between T and R is :

Question

$\begin{array}{1 1} T \infty e^{-3R} \\ T \infty \frac{1}{R} \\ T \infty \frac{1}{R^3} \\ T \infty e^{-R} \end{array} $