Troy is trying to study, and his sister is in the next room, playing with some balloons. She counts "one, two, three," and just as she says "three" she pops one of the balloons with a pin. Each time a balloon pops Troy is startled and jumps. By the time his sister has popped six balloons Troy begins to cringe when he hears the counting start, before he actually hears the balloon pop. In this

example of classical conditioning, the cringing that Troy experiences when he hears his sister start counting is:

a. a conditioned response
b. an unconditioned stimulus
c. an unconditioned response
d. a conditioned stimulus

A