The ratio of specific heat at constant pressure to specific heat at constant volume $(\gamma)$ for a gas is given by $\gamma = 1 + \frac{2}{f}$,where $f$ is the number of degrees of freedom of a molecule of the gas. What is the ratio of $\gamma_{d}$ for a rigid diatomic gas to $\gamma_{m}$ for a monoatomic gas?

• A
  $\frac{14}{23}$
• B
  $\frac{25}{21}$
• C
  $\frac{21}{25}$
• D
  $\frac{23}{14}$