For a diatomic gas,let $\gamma_1 = \frac{C_p}{C_v}$ for rigid molecules and $\gamma_2 = \frac{C_p}{C_v}$ for diatomic molecules that also have a vibrational mode. Which of the following options is correct? ($C_p$ and $C_v$ are the specific heats of the gas at constant pressure and volume,respectively.)

• A
  $\gamma_2 > \gamma_1$
• B
  $\gamma_2 = \gamma_1$
• C
  $2 \gamma_2 = \gamma_1$
• D
  $\gamma_2 < \gamma_1$