单选题：阅读文章，回答问题：CMost people feel lonely sometimes, but it usually

阅读文章，回答问题：
C
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. Psychologists are studying this phenomenon in an attempt to better understand long-term loneliness. These researchers have already identified 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. For example, 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. Situational loneliness is easy to understand and to predict.
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 loneli-ness have problems socializing. Unfortunately, many chronically lonely people think there is little or nothing they can do to improve their condition.
Many researchers agree that the loneliest people are between the ages 18 and 25, so a group of psychologists decided to study a group of college students. They found that more than 50% of the students were situational lonely at the beginning of the term as a result of their new circumstances, but had adjusted after a few months. Thirteen percent were still lonely after seven months due to shyness and fear. They felt very uncomfortable meeting strangers, even though they understood that their fear was not reasonable. The situational lonely students overcame their loneliness by making new friends, but the chronically lonely remained unhappy because they were afraid to do so.
Psychologists are trying to find ways to help habitually lonely people for two reasons. First of all, they are unhappy and unable to socialize. Secondly, researchers have found a connection be-tween chronic loneliness and serious illness such as heart disease. While temporary and situational loneliness can be a normal, healthy part of life, chronic loneliness can be a very sad, and some-times dangerous condition.
What does the passage mainly talk about? A．Loneliness and illnesses.
B．Three kinds of loneliness.
C．Loneliness and shyness.
D．Three causes of loneliness.