What Is the Reason That the Edge of the Sun's

What is the reason that the edge of the Sun's visible disk is darker than the center?

A) The gases near the edge are in regions where stronger magnetic fields inhibit the emission of light.
B) One sees more deeply into the Sun near its edge, and the gas is cooler at the deeper levels.
C) One sees into shallower layers of the Sun near the edge where the gas is cooler and so emits less light.
D) One sees into shallower layers of the Sun near the edge. Because the gas is less dense there, it emits less light.