Two Pipes Closed at One End of Length L₁ and L₂

Two pipes closed at one end of length L₁ and L₂ are excited at their resonant frequencies.If the beat period is B_f Hz,then the velocity of sound is given by:

A) B_f $\times$ L₁ $\times$ L₂ / (4L₁ $\times$ 4L₂)
B) 4 $\times$ B_f $\times$ L₁ $\times$ L₂ / (4L₁ - 4L₂)
C) 16 $\times$ B_f $\times$ L₁ $\times$ L₂ / (4L₁ + 4L₂)
D) 4 $\times$ B_f $\times$ L₁ $\times$ L₂ / (L₁ -L₂)
E) 4 $\times$ B_f $\times$ L₁ $\times$ L₂ / (L₁ + L₂)