A New Toy Store with a Bright Yellow Sign Recently

A new toy store with a bright yellow sign recently opened near five-year-old Jeffrey's home.The first few times he was driven past it,he had no reaction to it.Then his parents took him to the store several times.Now,when Jeffrey is driven past the bright yellow sign,Jeffrey gets excited.According to Pavlov's theory of classical conditioning,the bright yellow sign is the

A) unconditioned response.
B) conditioned response.
C) unconditioned stimulus.
D) conditioned stimulus.