A ray of light passes through four transparent media with refractive indices μ1 ,μ2 , μ3 and μ4 as shown in the figure. The surface of all media are parallel. If the emergent ray CD is parallel to the incident ray AB, then
(A) μ1 = μ2
(B) μ2 = μ3
(C) μ3 = μ4
(D) μ4 = μ1