A person eats a hamburger at a restaurant and develops a very bad stomachache after finishing eating. As a result of the sudden illness, the person cannot eat hamburgers anymore. Just thinking about them makes the person feel sick to the stomach. In this scenario, the thought of a hamburger is _________.
a. a conditioned stimulus
b. a negative reinforcer
c. an unconditioned response
d. an unconditioned stimulus
e. a conditioned response
