A recent study of several hundred female physicians showed that their tendency to develop coronary disease was inversely proportional to their dietary intake of two vitamins, folate and B6. The researchers concluded that folate and B6 inhibit the development of heart disease in women. Which one of the following would, if true, most weaken the researchers'conclusion? ['Physicians are more likely than nonphysicians to know a great deal about the link between diet and health.', 'The foods that contain significant amounts of the vitamins folate and B6 also contain significant amounts of nonvitamin nutrients that inhibit heart disease.', 'The physicians in the study had not been screened in advance to ensure that none had preexisting heart conditions.', 'It is very unlikely that a chemical compound would inhibit coronary disease in women but not in men.']