A survey has shown that public perceptions of comparative risks differ radically from the assessments of risk-management experts. For example, living near a nuclear power plant was judged a much greater risk by the survey respondents than it was by the experts. On the other hand, exposure to X-rays was judged a significantly lower risk by the survey respondents than it was by the experts. Psychologists have found that people tend to be more worried about risks they consider involuntary than about risks they consider voluntary. Thus the survey results were probably caused by the respondents' tendency to consider involuntary risks to be greater than they actually are. Which one of the following is an assumption required by the argument? ["The survey's respondents did not include people living near nuclear power plants.", 'Not all of the survey respondents considered living near a nuclear power plant to be a voluntary risk.', 'It is not the case that involuntary risks are usually greater than voluntary risks.', 'People never consider risks that they are greatly worried about to be lower than they actually are.']