Total Internal Reflection: the Critical Angle in Air for a Particular

Total internal reflection: The critical angle in air for a particular type of glass is 39.0°. What is the speed of light in this class glass? (c = 3.00 × 10⁸ m/s)

A) 1.97 × 10⁸ m/s
B) 1.94 × 10⁸ m/s
C) 1.91 × 10⁸ m/s
D) 1.89 × 10⁸ m/s
E) 2.00 × 10⁸ m/s