A lens forms a sharp image on a screen. On inserting a parallel sided glass slab between the lens and the screen, it is found necessary to move the screen a distance `d` away the lens in order for the image to be sharp again. If the refractive index of the material of the slab is `mu`, the thickness of the slab is
A. `mu d`
B. `(d)/(mu)`
C. `(mu - 1) (d)/(mu)`
D. `(mu d)/(mu - 1)`