$($d$)$ The measured molar heat capacity at constant volume, $C_v$ for a monatomic gas $($helium$,He)$ and a diatomic gas $($nitrogen$,N_2)$ at around room temperature and pressure are $12\mathrm{.}5J$ mole $K^{-1}$ and $20\mathrm{.}8Jmol{e}^{-1},$ respectively. Explain why the experimental value of $C_V$ for the diatomic molecule is lower than the estimated value whereas the experimental value of $C_V$ is close to the calculated value for the monatomic molecule. Under what conditions will the measured $C_V$ be close to the calculated $($classical$)$ value?