Premature babies who receive regular massages are more active than premature babies who do not. Even when all the babies drink the same amount of milk, the massaged babies gain more weight than do the unmassaged babies. This is puzzling because a more active person generally requires a greater food intake to maintain or gain weight. Which of the following, if true, best reconciles the apparent discrepancy described above? ['Massage does not increase the growth rate of babies over one year old, if the babies had not been previously massaged.', 'Increased activity causes the intestines of premature babies to mature more quickly, enabling the babies to digest and absorb more of the nutrients in the milk they drink.', 'Increased activity leads to increased levels of hunger, especially when food intake is not also increased.', "Massage increases premature babies' curiosity about their environment, and curiosity leads to increased activity."]