A prism of crown glass refracting angle of $50^{\circ}$ and mean refractive index $=1.51$ is combined with one flint glass prism of refractive index $=1.65$ to produce no deviation Then the angle of flint glass is

Question

$(a)\;3.92 ^{\circ} \\ (b)\;1.49^{\circ} \\ (c)\;2.67 ^{\circ} \\ (d)\;6.28^{\circ} $