A plate of thickness t, made of a material of refractive index μ, is placed in front of one slit in a double-slit experiment. The additional optical path difference and the minimum thickness required to obtain darkness at the centre are, respectively,
(a) (μ−1)t,2(μ−1)λ
(b) 2(μ−1)t,μ−1λ
(c) (μ−1)t,μ−1λ
(d) 2(μ−1)t,μ−1λ