A、Severe loneliness. B、Situational loneliness. C、Chronic loneliness. D、Temporary loneliness. A文中提及三种“孤独”，而未涉及severe loneliness，

Most people feel lonely sometimes. But it usually only lasts between a few minutes and a few hours. This kind of loneliness is not serious. In fact, it is quite normal. For some people though, loneliness can last for years. Now researchers say there are three different types of loneliness.
    The first kind of loneliness is temporary. This is the most common type. It usually disappears quickly and does not require any special attention. The second kind, situational loneliness, is a natural result of a particular situation--for example, a family problem, the death of a loved one, or moving to a new place. Although this kind of loneliness can cause physical problems, such as headaches and sleeplessness, it usually does not last for more than a year. The third kind of loneliness is the most severe. Unlike the second type, chronic loneliness usually lasts more than two years and has no specific cause. People who experience habitual loneliness have problems socializing and becoming close to others. Unfortunately, many chronically lonely people think there is little or nothing they can do to improve their conditions. Psychologists agree that one important factor in loneliness is a person’s social contacts, e.g. , friends, family members, co-workers, etc. We depend on various people for different reasons. For instance, our families give us emotional support, our parents and teachers give us guidance, and our friends share similar interests and activities. However, psychologists have found that, though lonely people may have many social contacts, they sometimes feel they should have more. They question their own popularity.