In a study of the effect of radiation from nuclear weapons plants on people living in areas near them, researchers compared death rates in the areas near the plants with death rates in areas that had no such plants. Finding no difference in these rates, the researchers concluded that radiation from the nuclear weapons plants poses no health hazards to people living near them. Which one of the following, if true, most seriously weakens the researchers' argument? ['Only a small number of areas have nuclear weapons plants.', 'Nuclear power plants were not included in the study.', 'Exposure to nuclear radiation can cause many serious diseases that do not necessarily result in death.', 'The researchers did not study the possible health hazards of radiation on people who were employed at the nuclear weapons plants if those employees did not live in the study areas.']