Recently we told you about a finding that more years of school could help students get higherscores on intelligence tests．That was the finding of a study of teenage males in Norway．Now，other research shows that physical activity may help students do better in their classes .
The research comes as educators in some countries are reducing time for activities like physical education．They are using the time instead for academic subjects like maths and reading．The studies appeared between 2007 and 2013．They included more than 55，000 children，aged 6 to 18．
Amika Singh：“Based on the results of our study，we can conclude that being physically active is beneficial for academic performance. There are，first，Physiological explanations，like more blood flow, and so more oxygen to the brain．Being physically active means there are more hormones(荷尔蒙)produced like endorphins(内啡肽)．And endorphins make your stress level lower and  your mood improved, which means you also perform better.”
Also，students involved in organized sports learn rules and how to follow them．This could improve their classroom behavior and help them keep their mind on their work．
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．
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

D．the research shows that the children aged 6 to 1 8 don't do sports

Which of the following is TRUE?

A．Being mentally active is good for academic subjects．

B．The more hormones you have，the more stressed you will get．

C．They have found many high-quality studies to prove the research．

D．The educators in the research think studies，not exercise，mean much to students．

The underlined sentence in Paragraph 7 probably means ＿＿＿＿.

A．good marks result from high-quality study

B．findings probably vary in different areas

C．physical activity will spread all over the world

D．academic performance depends on the surroundings

What's the purpose of writing the passage?

A．To call our attention to the sports at school．

B．To introduce all types of physical activities．

C．To represent the academic performance．

D．To improve students' health ．