A ray of monochromatic light is incident on one refracting face of a prism of angle 75°. It passes through the prism and is incident on the other face at the critical angle. If the refractive index of the material of the prism is 2 , the angle of incidence on the first face of the prism is
(a) 30°
(b) 45°
(c) 60°
(d) 0°