A Ray of Light Is Normally Incident on Face Ab $\alpha$

Multiple Choice

A ray of light is normally incident on face ab of a plastic prism with an index of refraction n = 1.20 as shown.
-Determine the largest value of the angle $\alpha$ so that the ray is totally reflected at the face ac if the prism is immersed in air.

A) 28°
B) 34°
C) 45°
D) 56°
E) Total internal reflection will not occur for any value of $\alpha$ .