If you want to improve your child's results at school, you could do a lot worse than ensuring that they do plenty of exercise. Scientists have already shown that physical activity can make you brainier. But a team in America has used scans to show that an important part of the brain actually grows in children who are fit. These youngsters tend to be more intelligent and have better memories than those who are inactive. Scientists also found that one of the most important parts of their brains was 12 percent larger than those of unfit children. They believe that encouraging children to take exercise from a very young age could help them do better at school later. Researchers from the University of Illinois, in the U.S., studied the brains of 49 children aged nine and ten using a magnetic resonance imaging scan  , a technique which provides very detailed pictures of organs and tissues in the body. They also tested the fitness levels of the children by making them run on a treadmill  . The scientists found that the hippocampus, a part of the brain responsible for memory and learning, was around 12 percent larger in the fitter children. Professor Art Kramer, who led the study published in the journal Brain Research, said the findings had important implications for encouraging individuals to take part in sport from a young age. "We knew that experience and environmental factors and socioeconomic status all impact brain development," he said. "If you get some terrible genes from your parents, you can't really fix that, and it's not easy to do something about your economic status. But here's something that we can do something about." All the following factors that have some influence on one's brain are mentioned EXCEPT   _  .
genetic factor economic status physical fitness economic development
Question: If you want to improve your child's results at school, you could do a lot worse than ensuring that they do plenty of exercise. Scientists have already shown that physical activity can make you brainier. But a team in America has used scans to show that an important part of the brain actually grows in children who are fit. These youngsters tend to be more intelligent and have better memories than those who are inactive. Scientists also found that one of the most important parts of their brains was 12 percent larger than those of unfit children. They believe that encouraging children to take exercise from a very young age could help them do better at school later. Researchers from the University of Illinois, in the U.S., studied the brains of 49 children aged nine and ten using a magnetic resonance imaging scan  , a technique which provides very detailed pictures of organs and tissues in the body. They also tested the fitness levels of the children by making them run on a treadmill  . The scientists found that the hippocampus, a part of the brain responsible for memory and learning, was around 12 percent larger in the fitter children. Professor Art Kramer, who led the study published in the journal Brain Research, said the findings had important implications for encouraging individuals to take part in sport from a young age. "We knew that experience and environmental factors and socioeconomic status all impact brain development," he said. "If you get some terrible genes from your parents, you can't really fix that, and it's not easy to do something about your economic status. But here's something that we can do something about." All the following factors that have some influence on one's brain are mentioned EXCEPT   _  .
A. genetic factor
B. economic status
C. physical fitness
D. economic development
Answer: