Adolescents who routinely engaged in moderate and energetic exercise showed long-term improvements in their academic performance,the British Journal of Sports Medicine study reported.
"Our study suggests that the effect of physical activity may be quite large," John Reilly,a professor at the University of Strathclyde said.The researchers looked at a sample of about 5000 children who were involved in a long-term study that tracks children born in the UK between 1991 and 1992.When children reached 11 years old,their daily physical activity levels were measured using an accelerometer for three to seven days.The device,similar to a pedometer（计步器）tracking the number of steps taken,recorded the average time children exercised,which was 29 minutes a day for boys and 18 minutes for girls.
"The actual levels of daily physical activity at age 11 were quite low," Mr.Reilly noted.The children had their academic performance tested at ages 11 and 13 with compulsory national tests for students,and also at 15 or 16 with the General Certificate of Secondary Education （GCSE） exam.The tests assessed the children's abilities in English,math and science subjects.
When factors that could affect academic performance including birth weight,mother's age at delivery and other socioeconomic factors were adjusted for,the results showed that the more children participated in moderate and energetic physical activity,the higher their test scores were at age 11 in all three subjects.For girls,science scores were most strongly linked to exercise. When they followed up with the kids at age 13，their academic scores were still linked to how much they had exercised when they were 11 years old.
By the time the youngsters took the GCSE exam,each 17-minute-per-day increase in physical activity for the boys was linked to an improvement in their scores.Every additional 12-minute increase a day in exercise for the girls was also linked to an increased score as well,especially in the science category.
The researchers have called for more studies to look at the possible academic benefits that could be gained if students exercise the recommended 60 minutes or more a day.



How did the researchers carry out the study？ ______

A. By asking different questions.
B. By using tracking equipment.
C. By asking teens to take some tests.
D. By asking teens to report the exercise time.

According to the passage,what can affect a teen's academic performance？ ______

A. His age.
B. His mother's weight.
C. His mother's age at delivery.
D. His birth date.

What does the underlined part in Paragraph 4 mean？ ______

A. When the children were tested again at age 13
B. When the kids continued physical exercise at 13
C. When they moved along with the 13-year-old kids
D. When they took an interest in the 13-year-old kids

According to the researchers,how long should the students exercise per day？ ______

A. 12 minutes
B. 18 minutes
C. 30 minutes
D. 60 minutes