$A$ vessel of depth $2d \ cm$ is half filled with a liquid of refractive index $\mu_1$ and the upper half with a liquid of refractive index $\mu_2$. The apparent depth of the vessel seen perpendicularly is

• A
  $d \left( \frac{\mu_1 \mu_2}{\mu_1 + \mu_2} \right)$
• B
  $d \left( \frac{1}{\mu_1} + \frac{1}{\mu_2} \right)$
• C
  $2d \left( \frac{1}{\mu_1} + \frac{1}{\mu_2} \right)$
• D
  $2d \left( \frac{1}{\mu_1 \mu_2} \right)$