While Out Trick-Or-Treating for Halloween,at First Billy's Neighbours Had to Remind

Multiple Choice

While out trick-or-treating for Halloween,at first Billy's neighbours had to remind him to say "trick or treat" before they dropped their treats into his bag.As he continued to visit houses in the neighbourhood,soon Billy would say "trick or treat" as soon as the neighbours answered the door.Which operant conditioning term is used to describe the cue provided by the neighbours answering the door

A)  discriminative stimulus
B)  conditioned stimulus 
C)  reward
D)  reinforcer