Dr. garcia is known for his difficult pop quizzes. typically, he closes the classroom door just before he's about to give a quiz, which makes his students anxious and tense. students soon learn to feel anxious whenever dr. garcia closes the classroom door. closing the door has become a(n):

Closing the door has become a conditioned stimulus.
The term conditioned stimulus comes from Pavlov and his experiment on dogs. Each time he wanted to feed the dog, he would ring a bell to let the dog know food was coming. After a while, the dog would start salivating each time it heard the bell because it knew it was going to be fed soon. The case above is similar - whenever Dr. Garcia closes the door, the students assume he will give them a pop quiz. 

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