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 μ 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 (μ - 1)t
(b) away from P by (μ - 1)t
(c) towards P by t(1 - 1/μ)
(d) away from P by t(1 - 1/μ)