A conditioned response can become extinct by reverse conditioning. That is, if a dog has been conditioned to drool when he hears a bell, he can be conditioned not to drool when he hears the bell.
Unconditioned response is unlearned and conditioned is learned.
When you smell your favorite food (unconditioned stimulus) you
become hungrey(unconidtioned response)
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Conditioned response occurs when brought about by a conditioned
stimulus.
Straight forward answer. I could add details but I'm sure you'll
understand the above answer.