The study leaves some questions unanswered，however．Ms．Stash says it is not possible to say whether the amount or kind of activity affected the level of academic improvement．This is because of differences among the studies .

　　Also．they were mostly observational studies．An observational study is where researchers do not do controlled comparisons．They only describe what they observe．So they might observe a link that students who are more active often have better grades．But that does not necessarily mean being active was the cause of those higher grades．

　　The researchers said they found only two high - quality studies．They called for more high-

　　quality studies to confirm their findings．They also pointed out that “outcomes for other parts of the world may be quite different. ”

　　Still，the general finding was that physically active kids are more likely to do better in school．Ms．Singh says schools should consider that finding before they cut physical education programs．Her paper on “Physical Activity and Performance at School”is published in the Archives of Pediatric and Adolescent Medicine．

　　72．The passage mainly tells us that ＿＿＿＿

　　A．a research on physical education has been done by the researchers

　　B．there exists a possibility that physical activity leads to higher grades

　　C．the amount or kind of activity directly affects academic level

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