After Repeatedly Pairing the Sound of a Bell with Food

Multiple Choice

After repeatedly pairing the sound of a bell with food being placed in a dog's mouth, the sound of the bell alone will make the dog salivate. At this point, the sound of the bell is a(n) :

A) conditioned stimulus (CS) .
B) unconditioned stimulus (UCS) .
C) unconditioned response (UCR) .
D) conditioned response (CR) .