Solana got a new puppy and she keeps the puppy's treats in a ceramic jar that makes a distinctive 'clunk' sound when the lid is put on or taken off. At first, the puppy comes when he sees or smells the food, but after a while, he only needs to hear the sound of the jar lid coming off to come running. In this example, the puppy's initial coming to the smell of the food is the:
a) Conditioned stimulus.
b) Unconditioned stimulus.
c) Conditioned response.
d) Unconditioned response.