Answer: C
Explanation: A good experiment does everything it can to eliminate confounding, or the influence of external sources on the data being measured. Orange juice could be a factor that plays a role in curing the symptoms, so provide a group with and without orange juice gives you the ability to show that the influence of orange juices didn’t have an effect on the cold, or did, but it gives you a baseline to show the effects of the medicine with and without orange juices.