A recent survey in a key middle school shows that high school students in this school have a special preference for playing football, far more than other balls. The survey also found that students who regularly play football do better than those who do not. It can be seen that playing football regularly can improve students' academic performance.
With the consent of the students' parents, the school has made a rule that only high school students whose academic performance is in the top 30% of the grade can often play football weakens the above conclusion most.