Reducing stress lessen 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 anaesthesia 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?

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