Kate's dog, Chewbarka, loves going on walks. Chewbarka has learned that when Kate grabs the leash, they are going for a walk. Kate starts asking Chewbarka, "Do you want to go for a walk?" before grabbing the leash. Now Chewbarka becomes excited when Kate asks this question, knowing it means they are going for a walk. Chewbarka's excitement in response to Kate's question is an example of:
A spontaneous recovery.
B stimulus generalization.
C higher-order conditioning.
D stimulus discrimination.