Editorial: A proposed new law would limit elementary school class sizes to a maximum of 20 students. Most parents support this measure and argue that making classes smaller allows teachers to devote more time to each student, with the result that students become more engaged in the learning process. However, researchers who conducted a recent study conclude from their results that this reasoning is questionable. The researchers studied schools that had undergone recent reductions in class size, and found that despite an increase in the amount of time teachers spent individually with students, the students' average grades were unchanged. Which one of the following is an assumption required by the researchers' argument? ['The only schools appropriate for study are large elementary schools.', 'Teachers generally devote the same amount of individualized attention to each student in a class.', 'Parental support for the proposed law rests solely on expectations of increased student engagement in the learning process.', "Degree of student engagement in the learning process correlates well with students' average grades."]