Scenario i

tyler is physically dependent on heroin and uses it intravenously multiple times per day. most often he uses with his dealer in a drug house on his street. one day when his dealer was out of town, tyler met a group of fellow users going to a drug party about two hours away. tyler tagged along, and noted that he felt odd injecting the drug in this setting, because the new people and environment was a marked departure from his usual routime. minutes later, although the drug was of the same quality and he took no more or less than he usually did, tyler overdosed and had to be rushed to the emergency room.
reference: ref 7-1

(scenario i) several decades ago, the psychologist sheppard siegel first noted the role that classical conditioning plays in drug overdose. in this model, the unconditioned stimulus in the scenario is and the conditioned stimulus is
a. the usual drug-taking environment; heroin

b. the new environment; heroin

c. heroin; the usual drug-taking environment

d. heroin; the new environment