Learning questions

Dr. Walsh is interested in seeing whether symptoms of depression can be manipulated using principles of classical conditioning. For several weeks of an experiment, he gives a group of patients a sweetened soda that has a mood-enhancing drug in it and notices that symptoms of depression improve significantly. Then, he removes the drug from the beverage and notices that the symptoms are still improved when the patients consume the soda. Which of the following is the conditioned stimulus in Dr. Walsh’s experiment?