If a diagram is shown such that an incident ray AO iclined at an angle of 50 degree to the interface CB, the refracted ray CB is found to lie along the surface. What is the refractive index of the medium X with respect to air?

This is how the diagram looks like:
A horizontal line COB is drawn. An incident ray AO which strikes the mirror at O, made an angle of 50 degree with the horizontal line being the interface at O. The refracted ray CB is in line with line COB. X is the medium.

A. sin50/sin40 B. sin40/sin50 C. sin90/sin50
D. sin40/sin90
E. sin90/sin40
I need working and explanation please!