A ray of light through four transparent media with refractive indices ${\mu }_{1,} {\mu }_{2, } {\mu }_{3, }and {\mu }_{4, }$ is shown in fig. The surfaces of all media are parallel. If the emergent ray CD is parallel to the incident ray AB, we must have

• A

  ${\mu }_1 ={\mu }_2$

• B

  ${\mu }_2 ={\mu }_3$

• C

  ${\mu }_3 ={\mu }_4$

• D

  ${\mu }_4 ={\mu }_1$