The Center of the Disk of the Visible Sun Appears

The center of the disk of the visible Sun appears brighter than the edges because we see:

A) less radiation from the cooler chromosphere near the edges of the Sun.
B) into deeper and hotter layers at the center of the disk.
C) into deeper and cooler layers at the center of the solar disk.
D) a greater contribution from the corona of the Sun at the center of the disk.