Mr. Lynch has always used his "mean face" to stop undesirable behavior in his first graders. However, even though he looks at Grayson with this face each time Grayson talks out of turn, Grayson is talking more than ever now. Of the following, which is the best description of the "mean face" for Grayson? 

a. It is a positive reinforcer. 

b. It is a negative reinforcer. 

c. It is a presentation punisher. 

d. It is a conditioned stimulus.