John Has to Give His Cat Garfield a Pill Every

John has to give his cat Garfield a pill every day. He always opens up the pill bottle (which makes a popping sound) and removes one pill before giving it to Garfield, and he follows this by giving him a treat as a reward. After a while, John notices that when he opens up the pill bottle, Garfield comes running to him right away. In this example, the __________ is the conditioned stimulus.

A) sound of the pill bottle opening
B) pill-taking
C) cat running to John
D) treat