The molar heat capacity, ${\mathrm{C}}_{\mathrm{v}}$ of helium gas is 3/2 R and is independent of temperature. For hydrogen gas, ${\mathrm{C}}_{\mathrm{v}}$  approaches 3/2 Rat very low temperature, equals 5 / 2 R at moderate temperatures and is higher than 5 / 2 R at high temperature. Choose the correct reason for the temperature dependence of ${\mathrm{C}}_{\mathrm{v}}$ in case of hydrogen: