A physically active lifestyle has been shown to help increase longevity. In the Wistar region of Bellaria, the average age at death is considerably higher than in any other part of the country. Wistar is the only mountainous part of Bellaria. A mountainous terrain makes even such basic activities as walking relatively strenuous; it essentially imposes a physically active lifestyle on people. Clearly, this circumstance explains the long lives of people in Wistar. Which of the following, if true, most seriously weakens the argument? ['Many people who live in the Wistar region have moved there in middle age or upon retirement.', 'Per capita spending on recreational activities is no higher in Wistar than it is in other regions of Bellaria.', 'In Bellaria all medical expenses are paid by the government, so that personal income does not affect the quality of health care a person receives.', 'The Wistar region is one of Bellaria, s least populated regions.']