题目选项分析

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?

【选项】From 1980 to 1990 the difference in average income between employees with advanced degrees and those with bachelor's degrees also increased.