Dr. Garcia 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, and his students begin to feel anxious and tense. Students soon learn to anticipate a pop quiz and feel anxious whenever Dr. Garcia closes the classroom door. Closing the door has become a(n): .
a) conditioned stimulus
b) unconditioned stimulus
c) unconditioned response
d) conditioned response