An ink mark is made on a piece of paper on which a glass slab of thickness $t$ is placed. The ink mark appears to be raised up through a distance $x$ when viewed at nearly normal incidence. If the refractive index of the material of the glass slab is $\mu$,then the thickness of the glass slab is given by:

• A
  $\frac{\mu x}{\mu-1}$
• B
  $\frac{(\mu-1)}{\mu}$
• C
  $\frac{\mu-1}{\mu x}$
• D
  $\frac{\mu}{(\mu-1) x}$