[ Physics ] Open Question : a material has an index refraction of 1.75. what is the critical angle ?

whats the critical angle of this material??? A. 42.8 b. 16.7 C. 76.4 D. 55.0 E. 49.2 oh and if the critical angle for cubic zirconium is 29.2 degrees, what the index of refraction?? A.1.00 B.1.33 C. 1.50 D.1.85 E. 2.05