If a ray of light in a denser medium strikes a rarer medium at an angle of incidence `i`, the angle of reflection and refraction are respectively r and r', if the reflected and refracted rays are at right angles to each other, the critical angle for the given pair of media is

Options

A. `tan^-1(sin i)`
B. `cot^-1(tan i)`
C. `sin^-1(tan r')`
D. `sin^-1(tan r)`.