What Causes Limb Darkening

What causes limb darkening?

A) The photosphere at the edge of the Sun's surface is cooler than it is in the middle of the Sun's surface.
B) The limb of the Sun is darker than the center because sunspots collect along the limb.
C) Light reaching Earth from the limb of the Sun originates in the higher, cooler layers of the Sun.
D) Convection within the Sun is more efficient laterally than it is vertically with the result that the middle latitude regions of the Sun's surface are hotter than the poles.