You are watching a rat in a Skinner box, and every time a red light comes on the rat presses the lever in the box. However, when a green light comes on the rat never presses the lever. In this case, it appears that the color of the light is acting as:_______.
a. a positive reinforcer for bar pressing.
b. a negative reinforcer for bar pressing.
c. an unconditioned stimulus for bar pressing.
d. a discriminative stimulus for bar pressing.