P is a point on the axis of a concave mirror. The image of P, formed by the mirror, coincides with P. A rectangular glass slab of thickness t and refractive index `mu` is now introduced between P and the mirror. For the image of P to coincide with P again, the mirror must be moved
A. towards `P` by `(mu-1)t`
B. away from `P` by `(mu-1)t`
C. towares `P` by `t(1-1//mu)`
D. away from `P` by `(1-1//mu)`
