单选题 .  A Census Bureau (人口调查局) survey released Thursday shows a college graduate can expect to earn $ 2.1 million working full-time between ages 25 and 64, which demographers (人口学家) call a typical work-life period. A master's degree-holder is projected to earn $ 2.5 million, while someone with a professional degree, such as a doctor or lawyer, could make even more than $4.4 million. In contrast, a high school graduate can expect to make $1.2 million during the working years, according to the bureau report that tracked the influence of education on lifetime earnings.
    Not all students look at college as an investment, "but I'm sure parents do," said Jacqueline King, policy analyst with the American Council of Education, a higher education advocacy (拥护) group. "The challenge is to convince those high school students on the margins that it is really worth their time to go to college." Kevin Malecek, a graduate student in American politics at American University in Washington, said most of his classmates find higher education to be worth the time and financial commitment. "They go to every single class, and they are trying to get the most out of their own dollar," he said. The survey was conducted between March 1998 and March 2000. All estimates are based on 1999 salaries and probably will increase as salaries rise over time, Census Bureau analyst Jennifer Day said. The estimates do not account for inflation (物价上涨) or for differences in the earning potential of various fields of study. For example, people with computer science degrees tend to earn more than those with social work degrees.6.  The phrase "students on the margins" in Paragraph 2 most probably refers to ______.