The figure shows a double-slit experiment. Each slit has a width $W$. $A$ thin glass slab of thickness $\delta$ and refractive index $\mu$ is placed in front of one of the slits. The intensity at the central point $C$ is measured as a function of the thickness $\delta$. At what thickness $\delta$ will the intensity at $C$ be minimum?

• A
  $\frac{(2n - 1)\lambda}{2}$
• B
  $\frac{\lambda}{2(\mu - 1)}$
• C
  $\frac{(2n - 1)\lambda}{2(\mu - 1)}$
• D
  $\frac{2\lambda}{(\mu - 1)}$