Researchers have established that when people are mentally engaged, biochemical changes occur in the brain that allow it to act more effectively in cognitive areas such as attention and memory. This is true regardless of age. People will be alert and receptive if they are faced with information that gets them to think about things they are interested in. And someone with a history of doing more rather than less will go into old age more cognitively sound than someone who has not had an active mind. Many expert are so convinced of the benefits of challenging the brain that they are putting the theory to work in their own lives." The idea is no necessarily to learn to memorize enormous amounts of information, "says James Fozard, associate director of the National Institute on Aging." Most of us don’’t need that kind of skill. Such specific training is of less interest than being able to maintain mental alertness." Fozard and others say they challenge their brains with different mental skills, both because they enjoy them and because they are sure that their range of activities will help the way their brains work. Gene Cohen, acting director of the same institute, suggests that people in their old age should engage in mental and physical activities individually as well as in groups. Cohen says that we are frequently advised to keep physically active as we age, but older people need to keep mentally active as well. Those who do are more likely to maintain their intellectual abilities and to be generally happier and better adjusted." The point is, you need to do both," Cohen says," Intellectual activity actually influences brain-cell health and size." What is the passage mainly about

A. How biochemical changes occur in the human brain.
B. Why people should keep active not only physically but also mentally.
C. How intellectual activities influence brain-cell health.
D. Why people should receive special mental training as they age.