Sandy Had Learned That If Her Mother Was Smiling at Her

Multiple Choice

Sandy had learned that if her mother was smiling at her when she came into the kitchen,it meant that Sandy would probably be given a treat to eat if she asked nicely.But if her mother was frowning,she would not give Sandy anything and instead would shoo her away.Sandy's mother's facial expression was serving as a __________.

A) conditioned stimulus.
B) discriminative stimulus.
C) positive reinforcer.
D) negative reinforcer.