When a thin transparent plate of thickness t and refractive index $\mathrm{\mu }$ is placed in the path of one of the two interfering waves of light, then the path difference changes by  

• A

  $(\mathrm{\mu }+1)\mathrm{t}$

• B

  $( \mathrm{\mu }– 1)\mathrm{t}$

• C

  $\frac{(\mathrm{\mu }+1)}{\mathrm{t}}$

• D

  $\frac{(\mathrm{\mu }-1)}{\mathrm{t}}$