A ray of light passes through four transparent media with refractive indices $\mu_1,\mu_2,\mu_3$ of all media are parallel, If the emergent ray CD is parallel ti incident ray AB. We must have

Question

 

$(a)\;\mu_1 = \mu_2 \\ (b)\;\mu_2=\mu_3 \\ (c)\;\mu_3=\mu_4 \\ (d)\;\mu_4 =\mu_1 $