Thermodynamics: JEE Main 2015 Physics question

Consider an ideal gas confined in an isolated closed chamber. As the gas undergoes an adiabatic expansion, the average time of collision between molecules increases as ${V^q},$ where $V$ is the volume of the gas. The value of $q$ is: $\left( {\gamma = {{{C_p}} \over {{C_v}}}} \right)$