In Young's double slit experiment, two slits $S_1$ and $S_2$ are $d$ distance apart and the separation from slits to screen is $D$. If two transparent slabs of equal thickness $t = 0.1 \text{ mm}$ but refractive indices $\mu_1 = 1.51$ and $\mu_2 = 1.55$ are introduced in the path of the beam $(\lambda = 4000 \text{ Å})$ from $S_1$ and $S_2$ respectively, the central bright fringe will shift by how many fringes?

• A
  $5$
• B
  $10$
• C
  $15$
• D
  $20$