The molar specific heat of an ideal gas at constant pressure and constant volume is $C_p$ and $C_v$ respectively. If $R$ is the universal gas constant and the ratio of $C_p$ to $C_v$ is $\gamma$,then $C_v$ is equal to:

• A
  $\frac{\gamma-1}{R}$
• B
  $\frac{1-\gamma}{1+\gamma}$
• C
  $\frac{1+\gamma}{1-\gamma}$
• D
  $\frac{R}{\gamma-1}$