A device having a transmitter and a receiver on-board is moving with speed $V$ between two walls as shown in the figure. The transmitter sends out signal of frequency $f_s$ and speed $C$ towards both the walls whereas the receiver is designed to detect the frequencies of echoes from the walls. Assuming that the walls are sufficiently away from the device to allow for multiple reflections, the frequency which will NOT be detected by the device is

• A

  $f_s\left(\frac{C+V}{C-V}\right)$
• B

  $f_s\left(\frac{C-V}{C+V}\right)$
• C

  $f_s$
• Correct answer
  D

  $f_s\left(\frac{C}{C-V}\right)$

Hence, the correct answer is option: 'D'.