Please answer a question about the following article about Affirmative action in the United States:

A study in 2007 by Mark Long, an economics professor at the University of Washington, demonstrated that the alternatives of affirmative action proved ineffective in restoring minority enrollment in public flagship universities in California, Texas, and Washington. More specifically, apparent rebounds of minority enrollment can be explained by increasing minority enrollment in high schools of those states, and the beneficiaries of class-based (not race) affirmative action would be white students. At the same time, affirmative action itself is both morally and materially costly: 52 percent of white populace (compared to 14 percent of black) thought it should be abolished, implying white distaste of using racial identity, and full-file review is expected to cost the universities an additional $1.5 million to $2 million per year, excluding possible cost of litigation.

How were rebounds of minority enrollment explained in Long's report?
increasing minority enrollment in high schools