In a study of patients who enrolled at a sleep clinic because of insomnia, those who inhaled the scent of peppermint before going to bed were more likely to have difficulty falling asleep than were patients who inhaled the scent of bitter orange. Since it is known that inhaling bitter orange does not help people fall asleep more easily, this study shows that inhaling the scent of peppermint makes insomnia worse. Which one of the following, if true, most seriously weakens the argument above? ['Some of the patients who enrolled in the sleep clinic also had difficulty staying asleep once they fell asleep.', 'Several studies have revealed that in many cases inhaling certain pleasant scents can dramatically affect the degree to which a patient suffers from insomnia.', 'Because the scents of peppermint and bitter orange are each very distinctive, it was not possible to prevent the patients from knowing that they were undergoing some sort of study of the effects of inhaling various scents.', 'The patients who inhaled the scent of bitter orange were, on average, suffering from milder cases of insomnia than were the patients who inhaled the scent of peppermint.']