Answer:
A. The words "good dog"
Explanation:
In this case, we know that the primary reinforcer is the treat. The dog is gaining an immediate reward for doing what you wanted it to do. But even after it, you also praise the dog. This praise is the secondary reinforcement. The secondary reinforcement will allow the trainer to deliver reinforcement even if the dog is not hungry, which might cause problems if you only use the primary reinforcement.
Answer:
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