Two strings $A$ and $B$ produce beats of frequency $\Delta f_1 > 0$. The tension in string $A$ is slightly increased and the beat frequency is found to be $\Delta f_2 > 0$. If the original frequency of $A$ is $f_0$ and $\Delta f_2 < \Delta f_1$, then the frequency of $B$ is

• A
  $f_0 + \Delta f_1$
• B
  $f_0 + \Delta f_1 - \Delta f_2$
• C
  $f_0 - \Delta f_1$
• D
  $f_0 + \frac{(\Delta f_1 + \Delta f_2)}{2}$