单选题 .  Moods, say the experts, are emotions that tend to become fixed, influencing one's outlook for hours, days or even weeks. That's great if your mood is a pleasant one, but 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. Modem pharmacology (药物学) offers a lot of tranquilizers (镇静剂), antidepressants 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 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 drugstore, try the following approach.
    Of all the mood-altering self-help techniques, aerobic(增氧健体的) exercise seems to be the most efficient 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 compared 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 tamed activities that boost the heart rate, increase circulation(血液循环) and improve the body's utilization(利用) of oxygen. Do them for at least twenty minutes a session, three or five times a week.1.  What is the main subject of the passage? ______