A recent research shows that physical activity(体育活动) may help students do better in their classes. The research comes because educators in some countries are reducing time for activities like P.E. class.
Researchers go over the results of fourteen studies. Twelve were from the United States, one from Canada and one from South Africa. The studies appeared between 2009 and 2013. They included more than 55,000 children, aged six to eighteen.
Thomas Singh, one of the researchers, says the studies showed a relationship between physical activity and scores on subjects such as math, English and reading. Ms. Singh offers some possible explanations (解释):
“ There are, first, physiological (生理的) explanations, like more blood flow, and so more oxygen (氧气) to the brain. Being physically active means there are more hormones（ 荷尔蒙） produced like endorphins ( 脑内啡). And endorphins make your stress level lower and your mood improved, which means you will perform better in class. Also, students who take part in organized sports learn rules and how to follow them. This could improve their classroom behavior and help them know what they should do.
All in all, the finding tells us that physically active kids are more likely to do better in school. Ms. Singh says schools should consider the finding before they cut physical education programs. 
What led the researchers to do the studies?

A．Schools’cutting physical activity.

B．Their interest in physical activity.

C．The tests including about 55,000 children.

D．Kids’unsatisfying schooling performance.

How many countries took part in the studies?

A．14.

B．12

C．2.

D．3.

Which of the following is NOT one of the advantages of physical activity according to the passage?

A．Getting more blood flow.

B．Better classroom behavior.

C．Feeling relaxed and refreshed.

D．Greater ability to organize activities.