According to a recent cross-cultural study, married people in general have longer life expectancies than do people who divorce and do not remarry. This fact indicates that the stress associated with divorce adversely affects health. Which of the following, if true, points to a weakness in the argument above? ['Adults who have never married have shorter life expectancies than do married people of the same age.', 'Overall life expectancies differ among countries, even among countries with similar cultures.', 'People often show signs of stress when undergoing a divorce.', 'Life expectancy varies with age-group, even among married people.']