a sleep study by Sculling (2019) in which students were given extra credit to sleep 8 or more hours per night. Half the students succeeded in sleeping 8+ hours per night and they did significantly better on the final exam compared to students who slept less than 8 hours per night. What aspect of this study allows for a strong conclusion that more sleep actually caused students to perform better academically? a. Students who slept 8+ hours per night already had better grades to begin with b. After controlling for any differences in previous grades, 8-hour sleepers still did better on the final exam c. The study had only 34 participants d. The amount of sleep per night was measured by student self-report