Researcher: Results indicate that the higher their educational level, the better are students' mathematical skills. These results do not prove that education improves mathematical skills, however, since it is possible that students who have better mathematical skills to start with are the students who reach higher educational levels. The reasoning of the researcher's argument is most similar to that of which one of the following arguments? ['Results indicate that the quality of papers submitted for publication varies significantly from university to university. This may say nothing about the quality of education offered at different schools, but may only reflect a defect in the review process.', "Results from competition indicate that professional athletes outperform amateur athletes. These results do not prove that becoming a professional athlete improves one's athletic performance, since it is possible that the athletes who become professionals are those whose performance is better to begin with.", 'Studies suggest that some people who are gifted in higher mathematics are inept at performing simple arithmetical calculations. These studies do not show that being good at mathematics precludes being good at arithmetic, since there are also many people who are good at both.', 'Surveys indicate that politicians with law degrees are better at what they do than politicians without law degrees. These surveys do not prove that having a law degree makes one a better politician, since it is possible that many politicians without law degrees were left out of the survey.']