A、Because it gives people a sense of achievement. B、Because most people like doing it every day. C、Because it is neither biochem

Moods, say the experts, are feelings that are likely to be fixed, having effects on one’s outlook for hours, days, weeks or even years. That’s great if your mood is a pleasant one, but it will be a problem if you are sad, anxious, angry or simply lonely.
    Perhaps the best way to deal with such moods is to talk them out: sometimes, though, there is no one to listen. Modern pharmacy offers a lot of anti-anxiety drugs. [20]What many people don’t realize, however, is that scientists have discovered the effectiveness of several non-drug methods to make you free from an unwanted mood. These can be just as useful as drugs, and have the added benefit of being non-poisonous. So next time you feel out of sorts, don’t head for the drug store—try the following method.
     [20]Of all the mood-changing self-help techniques, aerobic exercise seems to be the best cure for a bad mood. "If you could keep the exercise, you’d be in high spirits," says Kathryn Lance, author of Running for Health and Beauty. Researchers have explained biochemical and various other changes that make exercise seem preferable to drugs as a mood-raiser. Physical work such as housework, however, does little. The key is aerobic exercise—running, cycling, walking, swimming, or other repetitive activities that increase the heart rate and circulation, and improve the consumption of oxygen. You may do them for at least 20 minutes a time, three to five times a week.
19. What is the effect of moods on people?
20. What is the best way that most people don’t realize to keep a good mood?
21. According to the speaker, why is aerobic exercise able to cheer people up?