Which of the following is true about photoreceptors?
a. rods absorb light throughout the visual spectrum but confer only gray tone vision.
b. cones are more numerous than rods in the eye.
c. rods are used in high light environments.
d. each rod is connected with only one bipolar cell and one ganglia cell.

a. Rods absorb light throughout the visual spectrum but confer only gray tone vision.

Explanation:

Rods and cones are the two types of photoreceptors present in the eyes. Rods are responsible for night vision or the vision in dim light. Rods do not provide color vision. Therefore, only black, white, and all shades of gray are seen in dim light.  

In complete darkness, full regeneration of cone photopigments is delayed and the eyes are adapted to the darkness first. Rhodopsin is the pigment of rods. It regenerates more slowly and can detect all the colors of the light but does not provide color vision. Therefore, irrespective of the detection of a much dimmer light, threshold flashes appear gray-white only because only the rods are functioning.

b brown mice / white sand dunes