Consider a gas with molar mass $M$. If sound at frequency $f$ is introduced to a tube of this gas at temperature $T$, an internal acoustic standing wave is set up with nodes separated by $L$. The adiabatic constant $\gamma = \frac{C_p}{C_v}$ is

• A
  $\frac{M f^2 L^2}{R T}$
• B
  $\frac{M f^2 L^2}{4 R T}$
• C
  $\frac{4 M f^2 L^2}{R T}$
• D
  $\frac{3 M f^2 L^2}{2 R T}$