For one academic year all the students at a high school were observed. The aim was to test the hypothesis that studying more increased a student' s chances of earning a higher grade. It turned out that the students who spent the most time studying did not earn grades as high as did many students who studied less. Nonetheless, the researchers concluded that the results of the observation supported the initial hypothesis. Which one of the following, if true, most helps to explain why the researchers drew the conclusion described above? ['In each course, the more a student studied, the better his or her grade was in that course.', 'The students who spent the least time studying tended to be students with no more than average involvement in extracurricular activities.', 'Students who spent more time studying understood the course material better than other students did.', 'The students who spent the most time studying earned higher grades than did some students who studied for less time than the average.']