资料题：What is the matter in cigarettes that causes cancer（　）。

Why do people like smoking? One reason is that people become addicted to cigarettes.To be addicted means that your body comes to need cigarettes.The drug that gets people into the habit of smoking cigarettes is nicotine.When people smoke, the nicotine (尼古丁) goes right into the blood and makes people feel comfortable.A smoker's body gets used to the nicotine and if he stops smoking he feels nervous.Many smokers try to stop smoking but because of the need for nicotine they feel so uncomfortable that they often find it too difficult to stop.Another reason is that people simply enjoy smoking.Having a cigarette for many people means taking a break.For some people smoking becomes part of their daily life.Many people enjoy smoking because it gives them something to do with their hands.Many people also like the taste of tar in cigarettes.However, it is the tar that causes cancer. While government and health experts have tried to get people to give up smoking entirely, cigarette makers have tried to keep selling them by making cigarettes with less tar.Many people in western countries have welcomed these cigarettes since they find it hard to stop smoking but they want to reduce the danger to their health.