A、Smoke. B、Tar. C、Nicotine. D、Ashes. C短文开头便介绍了香烟使人上瘾的主要原因在于其成分尼古丁。故选C。

Smoking can be harmful to your health. But why do people smoke?
    One reason is that people become addicted to cigarettes. To be addicted means that your body comes to need them. The addictive substance in cigarettes is nicotine. When people smoke, the nicotine goes right into the blood stream and makes them feel relaxed. A smoker’s body gets accustomed to the nicotine and if he stops smoking he feels nervous. Many smokers try to stop smoking, but because of the addiction to nicotine they feel so uncomfortable that they often find it too difficult to stop.
    Another reason is that people simply enjoy smoking and what it symbolizes. Having a cigarette for many people means taking a break. For some people smoking becomes part of certain social rituals, for example, the cigarette after dinner. Many people enjoy smoking because it gives them something to do with their hands. Reaching for a cigarette, lighting it, flicking the ashes are especially comforting in situations where a person feels tense.
    Many people also like the taste of tar in cigarettes. However, it is the tar that causes cancer. While governments and health experts have tried to get people to give up smoking entirely, cigarette manufacturers have tried to keep selling them by producing cigarettes with less tar. Many people in Western countries have welcomed these cigarettes since they find it hard to stop smoking but want to reduce the risk to their health.
19 What is the addictive substance in cigarettes?
20 Why do many smokers find it too difficult to stop smoking?
21 What makes cigarettes with less tar welcomed by many people in Western countries?