The difference in average annual income in favor of employees who have college degrees, compared with those who do not have such degrees, doubled between 1980 and 1990. Some analysts have hypothesized that increased competition between employers for employees with college degrees drove up income for such employees. Which of the following, if true, most seriously undermines the explanation described above? ['The unemployment rate changed very little throughout the 1980s', 'During the 1980s there were some employees with no college degree who earned incomes comparable to the top incomes earned by employees with a college degree.', 'During the 1980s a growing percentage of college graduates, unable to find jobs requiring a college degree, took unskilled jobs.', 'The average age of all employees increased slightly during the 1980s.']