A、Because they have more nerve cells. B、Because they have more brain chemicals. C、Because they have stronger muscles. D、Because

It’s clear that the brain benefits from exercise. William’s studies with rats have demonstrated two primary effects of activity: vigorous physical exercise provides the brain with more fuel, and skill-based exercise increases the formation of connections in the brain making the brain better able to process information.
    Human studies also suggest that regular exercise can improve the speed with which the brain processes information. A succession of scientific studies indicates that exercise can help improve balance and strength in elderly individuals.
    And children who engage in regular physical activity do better in school than their inactive classmates. Consequently, the brains of people who exercise may be better equipped to tackle mental challenges. Physical exercises increase the amount of certain brain chemicals that stimulate growth of nerve cells.
    Inactivity may also have negative effects on mind and body alike. Scientists recognize that mind is body, and body is mind. The most beneficial forms of exercise engage both.
Questions 21 - 25 are based on the passage you have just heard.
21. What makes the brain better able to process information?
22. What is the academic edge deriving from according to the passage?
23. What does the sentence "Mind is body, and body is mind" mean?
24. Why may those who exercise tackle mental challenges better?
25. What is the most beneficial form of exercise?