Perhaps the best way to deal with such moods is to talk them out; sometimes, though, there is no one to listen. Modern pharmacology(药理学)offers an abundance of tranquilizers(安眠药), anti-depressants and anti-anxiety drugs. What many people don't realize, however, is that scientists have discovered the effectiveness of several non-drug approaches to make you loose 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 approach.
Of all the mood-altering self-help techniques, aerobic(增氧健身的)exercise seems to be the most efficient cure for a had 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 compare favorably to drugs as a mood-raiser. Physical exertion such as housework, however, does little. The key is aerobic exercise--running, cycling, walking, swimming or other repetitive and sustained activities that boost the heart rate, increase circulation and improve the body's utilization of oxygen. Do them for at least 20 minutes a session, three to five times a week.
What is the main subject of the passage？

A. How to beat a bad mood.
B. How to do physical exercises.
C. How to talk bad moods out.
D. How to be involved in aerobic exercise.