A thin glass prism of unknown refractive index is tested in air and then completely immersed in water (nwater=1.33). Its apex angle is 4∘. The measured angular deviations are 2.0∘ in air and 0.50∘ in water. The refractive index of the glass is
a) 1.84
b) 1.43
c) 1.50
d) 1.63