Answer:
conditiond stimulus
Step-by-step explanation:¨Professor Kariag is turning off the projector to signal that there will be a quiz, so he is making it the conditioned stimulus.
Students experienced it previously so many times that they became aware that the signal means that a hard quiz was to come thereafter.
The act of just turning off the projector could have occurred at random , and that originally was just another neutral stimulus. After a neutral stimulus is paired with the doing of a quiz, a conditioned response is formed as well:
Students begin already to feel anxious.