Infant development researchers released a study recently that compared two groups of babies: those who were fed breast-milk during their first six months of life and those who were fed formula during the same period. The study's results showed that the group of infants fed breast-milk weighed significantly more on average at one year of age than did the group fed formula. The researchers concluded that a breast-milk diet leads to greater weight gain among babies than a formula diet. Which of the following would cast the most doubt on the researchers' conclusion? A. The mothers of infants in the breast-milk-fed group weighed significantly more on average than the mothers of babies in the formula-fed group. B. Many of the infants in the formula-fed group at one year of age weighed more than the average weight of the breast-milk fed group at one year of age. C. The average weight of the infants in the breast-milk-fed group was significantly greater than the average weight of the babies in the formula-fed group when the study began. D. In controlled laboratory studies, a hormone found in breast-milk has been linked to increased weight gain in baby mice. E. The average amounts of breast-milk and formula consumed by the two groups of infants over the course of the study were approximately equal.