A cavity contains blackbody radiation in equilibrium at temperaturel. the specific heat per unit volume of the photon gas in the cavity is of the form cp = t',

A cavity contains blackbody radiation in equilibrium at temperaturel. The specific heat per unit volume of the photon gas in the cavity is of the form Cp = T', where 7 is a constant. The cavity is expanded to twice its original volume and then allowed to equilibrate at the same temperature T. The new internal energy per unit volume is (a) 47 (b) 2T (C) T (d)4