Deirdre used to be shy and unwilling to speak out in class. Last semester, she was in a class that involved a lot of discussion groups, and she found that her classmates were willing to listen attentively to her ideas and often asked her for input when the discussion bogged down. Now Deirdre is more outspoken and confident during class discussions. Based on principles of conditioning, Deirdre's new outspoken, confident manner is MOST likely a result of _________.
A) spontaneous recovery B) extinction C) reinforcement D) punishment