Which of the following is true about photoreceptors?

a. if all cones are stimulated equally, all colors are absorbed by the cones and the color perceived is black.
b. rods absorb light throughout the visual spectrum but confer only gray tone vision.
c. in dim light, images are focused directly on the rods in the fovea centralis.
d. three types of color-sensitive photoreceptors exist: red, green, and yellow.