A diverging beam of light from a point source S having divergence angle alpha falls symetricaally on a glass slab as shown in fig.The angles of incidence of the two extreme rays are equal.If the thickness of the glass slab is t and the refractive index is n,then the divergence angle of the emergent beam is,

a)0

b)alpha

c)sin inverse 1/n

d)2 sin inverse 1/n