Fifty chronic insomniacs participated in a one-month study conducted at an institute for sleep disorders. Half were given a dose of a new drug and the other half were given a placebo every night before going to bed at the institute. Approximately 80 percent of the participants in each group reported significant relief from insomnia during the first two weeks of the study. But in each group, approximately 90 percent of those who had reported relief claimed that their insomnia had returned during the third week of the study. Which one of the following, if true, most helps to explain all the data from the study? ["Most insomniacs sleep better in a new environment, and the new drug has no effect on an insomniac's ability to sleep.", 'Some insomniacs cannot reliably determine how much sleep they have had or how well they have slept.', 'The psychological comfort afforded by the belief that one has taken a sleep-promoting drug is enough to prevent most episodes of insomnia.', 'The new drug is very similar in chemical composition to another drug, large doses of which have turned out to be less effective than expected.']