This is not enough to justify the researchers’ conclusion. It is possible that students who studied the most earned better grades than only a small proportion of the students who studied little, in which case their conclusion remains unexplained.

This does not imply that students studied less as the year progressed, only that they received lower grades. Without information about the time students spent studying, this does not support the researchers’ conclusion.

This states that students in a given course are more likely to succeed with more study. It is consistent with the evidence because it implies the students who studied the most were in different courses than the students who performed better with less studying.

This does not explain the researchers’ conclusion. It is not implied that involvement in extracurriculars makes a student more or less likely to earn high grades.

Researchers concluded that studying leads to higher grades, not a better understanding of the course material. This does not explain why some students who did not study got better grades than students who did.