Dr. Elliott exposes rats to a vanilla scent prior to receiving a food pellet in the left corner \n of their cage but provides no food after exposure to a lemon scent. After several trials, \n upon smelling a vanilla scent, the rats wait at the far-left corner of the cage regardless of \n whether a food pellet is present. However, they do not wait in the far-left corner when \n exposed to the lemon scent. What is the unconditioned stimulus in the experiment?