Sal's dog loves to go on a walk and starts spinning in circles and wagging his tail in excitement when his lead is clipped onto his collar. Eventually Sal begins to notice that his dog starts to act excited when he puts on his tennis shoes before a walk. His dog's behavior of spinning in circles and wagging his tail when Sal puts on his tennis shoes is a(n) __________.
◦ conditioned response
◦ conditioned stimulus
◦ unconditioned response
◦ unconditioned stimulus