Joey Likes to Watch Wrestling on TV, but His Mother

Joey likes to watch wrestling on TV, but his mother usually does not allow him to do this. However, Joey has noticed that when his mom has a bad day at work and gets very
Tired, she usually doesn't mind if he watches wrestling. As a result, Joey usually will only ask to watch wrestling if his mother has had a hard day at work. In this instance, the kind of day that Joey's mother has at work would be considered a:

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