At home, you rattle the chain on your dog’s leash every time you prepare to take him for a walk. After several episodes like this, you find that your dog comes running to the front door even when you pick up the leash to put it back in the closet. In this example, what is the conditioned stimulus?

a. the sound of the leash

b. going for a walk

c. the front door

d. the dog runs to the door