题目内容：
阅读下列短文，回答题。 Moods, say the experts, are feelings that are likely to become fixed, having effects on one' s out-look ( way of looking at things ) 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 noone to listen． Modern pharmacology (药物学) offers a lot tranquilizers (镇定剂 ) and anti-anxietydrugs． What many people don' t realize, however, is that scientists have discovered the effectivenessof several non-drug methods to make you free from an unwanted mood． These can be just as useful asdrugs, and have the added benefit of being nonpoisonous． So next time you feel out of sorts, don' thead for the drug store--try the following method．
Of all the mood-changing self-help techniques, aerobic exercise seems to be the best cure for abad mood． "If you could keep the exercise, you' d be in high spirits, "says Kathryn L．ance, author ofRunning for Health and Beauty．
Researchers have explained biochemical and various other changes that make exercise comparefavorably to drugs as mood-raiser． Physical work such as housework, however, does little． The key isaerobic exercise--running, cycling, walking, swimming, or other repetitive and sustained activitiesthat increase the heart rate and circulation(循环), and improve the body's use of oxygen． Do themfor at least 20 minutes a time,three to five times a week．
What is the main subject of the passage? A．How to beat a bad mood．
B．How to talk bad moods out．
C．How to do physical exercises．
D．How to do aerobic exercise．
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