In the Figure Below, One Can See That the Sun's

In the figure below, one can see that the Sun's surface appears darker towards the edges or "limbs" of the disk of the Sun.Why does this happen?

A) the chromosphere casts shadows on the edges of the Sun from our perspective.
B) the corona casts shadows on the edges of the Sun from our perspective.
C) the Sun is a higher temperature at the edges of the disk.
D) the viewing angle and lower temperature of the Sun's outer layers results in a dimming effect.
E) sunspots look much bigger when viewed edge-on.