An initially parallel cylindrical beam travels in a medium of refractive index $$\mu(I) = \mu_0 + \mu_2 I$$, where $$\mu_0$$ and $$\mu_2$$ are positive constants and $$I$$ is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius. The initial shape of the wave front of the beam is

A

convex

B

concave

C

convex near the axis and concave near the periphery

D

planar