Dietary researcher: A recent study reports that laboratory animals that were fed reduced-calorie diets lived longer than laboratory animals whose caloric intake was not reduced. In response, some doctors are advocating reduced-calorie diets, in the belief that North Americans' life spans can thereby be extended. However, this conclusion is not supported. Laboratory animals tend to eat much more than animals in their natural habitats, which leads to their having a shorter life expectancy. Restricting their diets merely brings their caloric intake back to natural, optimal levels and reinstates their normal life spans. Which one of the following, if true, would most weaken the dietary researcher's argument? ['North Americans with high-fat, low-calorie diets generally have a shorter life expectancy than North Americans with low-fat, low-calorie diets.', 'Not all scientific results that have important implications for human health are based on studies of laboratory animals.', 'North Americans, on average, consume a higher number of calories than the optimal number of calories for a human diet.', 'Some North Americans who follow reduced- calorie diets are long-lived.']