The good effect of physical exercise on skeletal muscle is manifested in many aspects: the increase of muscle volume. Physical exercise can promote blood circulation, strengthen the nutrition of skeletal muscle, make muscle cells get more nutrition, and make muscle cells thicker and larger, instead of increasing the number of muscle cells. The muscle mass of athletes, especially those in strength events such as weightlifting, is obviously greater than that of normal people. This shows that physical exercise and sports training can increase muscle mass. The effect of physical exercise on muscle volume is very obvious. Generally, only strength training can increase muscle volume, and what muscles are trained will increase muscle volume and muscle strength. It has been proved by a large number of experiments that physical exercise can increase muscle strength, and the effect of physical exercise on muscle strength is also very obvious. A few weeks of strength exercises will obviously increase muscle strength and muscle elasticity. People with good physical exercise habits often carry out some traction exercises during exercise, which can increase muscle elasticity, thus avoiding various sports injuries caused by severe muscle contraction in daily activities and physical exercise.

First, physical exercise can promote blood circulation, strengthen the nutrition of skeletal muscle, make muscle cells get more nutrition, and make muscle cells thicker and bigger, instead of increasing the number of muscle cells.

B. Exercise can thicken respiratory muscles, strengthen the ability of contraction and relaxation, enhance respiratory function, increase vital capacity and increase the number of alveoli involved in breathing.

C. Exercise can promote blood circulation and speed up blood flow, thus reducing the occurrence of cardiovascular diseases.

D, exercise can promote blood circulation and strengthen the delivery of nutrients, so it can improve brain nutrition, so it does not meet the meaning of the question.

So choose: a.