Reducing stress lessens a person' s sensitivity to pain. This is the conclusion reached by researchers who played extended audiotapes to patients before they underwent surgery and afterward while they were recovering. One tape consisted of conversation; the other consisted of music. Those who listened only to the latter tape required less anesthesia during surgery and fewer painkillers afterward than those who listened only to the former tape. Which one of the following is an assumption on which the researchers' reasoning depends? ['All of the patients in the study listened to the same tape before surgery as they listened to after surgery.', 'Both anesthesia and painkillers tend to reduce stress.', 'The psychological effects of music are not changed by anesthesia or painkillers.', 'Listening to music reduces stress.']