Dr. Meyer is known for his difficult pop quizzes. Immediately before he springs a pop quiz on his students, he typically goes to
the classroom door and closes it. Students soon learn to anticipate a pop quiz whenever Dr. Meyer closes the classroom door. Closing the door has become a
Conditioned stimulus is the factor that was at neutral state at previous time but after getting connected with unconditioned stimulus it produces a trigger in the form of conditioned response.
According to the question, door closing is conditioned stimulus because door gets combined with the closing action done by Dr. Meyer which generates a trigger that indicates students about pop quiz.
