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In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.

Article: Providing small classes for at least several grades starting in early primary school gives students the best chance to succeed in late grades, according to groundbreaking new research from a Michigan State University scholar. The research by Spyros Konstantopoulos, a professor of education, is the first to examine the effects of class size over a period and for all levels of students. The study appears in the American Journal of Education. He is also a member of a group for the Department of Education's Instiute of Education Sciences that will give official advice on class size to the states. He said the advice will mirror his research: the best plan is to provide continuous small classes(13 to 17 students) for at least several years starting in kindergarten or first grade. "For a logn time states thought they could just do it in kindergarten or first grade for one year and get the benefits," He said. "I don't believe that. I think you need at least a few years in a row where all students, and especially low-achievers, receive the treatment, and then you see the benefits later." His research used data from the Project Star study in Tennessee that analyzed the effects of class size on more than 11,000 students in primary and middle school. He found that students who had been in small classes from kindergarten through third grade had actually higher test scores in grades four through eight than students who been in larger classes early on. Students from all achievement levels benefited from small classes, the research found. "But low-achievers benefited the most, which narrowed the achievement gap with high -achievers in science, reading and math, " he said. Although the study didn't consider classroom practices, he said the reason for the narrowing gap is likely due to low-achieving students receiving more attention from teachers. " _ is especially important in poorer schools because teacher effectiveness matters more in schools with more disadvantaged and low-performing students, " he said.
Answer: the size of the class
Output:
The professor argues about _ .