Compared with those college students who do not play football, college students who often play football are generally healthier. It can be seen that football can exercise and improve physical health. Which of the following is true that would most weaken the above assertion? ['College students play football out of hobbies, not to exercise.', 'College students who are not in good health generally do not participate in intense football.', 'Football is also dangerous and easy to hurt.', 'Research shows that long-distance running is more effective than playing football.']