A survey of alumni of the class of 1960 at Aurora University yielded puzzling results. When asked to indicate their academic rank, half of the respondents reported that they were in the top quarter of the graduating class in 1960. Which one of the following most helps account for the apparent contradiction above? ['Academic rank at Aurora University was based on a number of considerations in addition to average grades.', 'Almost all of the alumni who graduated in 1960 responded to the survey.', 'Not all the alumni who were actually in the top quarter responded to the survey.', 'A disproportionately large number of high-ranking alumni responded to the survey.']