The reason for a "blind spot" in the visual field is that ______ select one:
a. the lens cannot focus all of the visual field onto the retina.
b. there are no photoreceptors in the retina where the axons exit the eye.
c. blood vessels collect together and enter the eye at the blind spot.
d. retinal cells die with age and overuse, resulting in blind spots.
e. rods are less sensitive to light than are cones.