It is probably not true that colic in infants is caused by the inability of those infants to tolerate certain antibodies found in cow' s milk, since it is often the case that symptoms of colic are shown by infants that are fed breast milk exclusively. Which one of the following, if true, most seriously weakens the argument? ["Symptoms of colic generally disappear as infants grow older, whether the infants have been fed breast milk exclusively or have been fed infant formula containing cow's milk.", "When mothers of infants that are fed only breast milk eliminate cow's milk and all products made from cow's milk from their own diets, any colic symptoms that their infants have manifested quickly disappear.", "Infants that are fed breast milk develop mature digestive systems at an earlier age than do those that are fed infant formulas, and infants with mature digestive systems are better able to tolerate certain proteins and antibodies found in cow's milk.", 'A study involving 500 sets of twins has found that if one infant has colic, its twin will probably also have colic.']