A Researcher Is Interested in the Effect of Teacher-Student Ratios

A researcher is interested in the effect of teacher-student ratios. She finds 10 fifth-grade classrooms with 30-40 students per class and 10 others with 15-25 students per class. She discovers that there is a negative correlation between class size and student achievement. Which one of the following conclusions can we draw from this study?

A) Although class size doesn't necessarily cause changes in achievement,class size can help us predict school achievement.
B) Classes should be as large as reasonably possible.
C) Classes should be as small as reasonably possible.
D) The researcher has conducted a descriptive study.