Question 18 is based on the passage you have just heard.

To help ourselves and others, it’s important to know something about drugs. A drug is a chemical substance. It can bring about a physical, emotional or mental change in people. Alcohol and tobacco are drugs. Caffeine, a substance found in coffee and some soft drinks is also a drug. (16) Drug abuse is the use of the drug legal or illegal that hurts the person or someone close to him. A drug user is the person who takes the drug. There are many kinds of drug users. Experimental users may try drugs once or twice. They want to see what the effects would be. (17)Recreational users take drugs to get high. They use drugs with friends or at parties to get into the mood of things. Regular users take drugs all the time. But they are often able to keep up with the normal routine of work. (18)Dependent users can’t relate to anything but drugs. Their whole life centers around drugs. They feel extreme mental or physical pain without drugs. It’s not always easy to tell if someone is using drugs. In the earlier stages, drug use is often hard to see. Sometimes people like drugs or need drugs so much they can’t do without them. They are dependent upon drugs. Only a few kinds of drugs can cause physical dependents, but almost any drug when it's misused, can make a person think he needs it all the time. By this time, it’s too late and the person is hooked.

(18). What does the speaker say about drug dependents?

解析：说到依赖性吸毒者，文中提到他们已经除毒品之外和其它任何东西都没关系了，且一生都离不开毒品了，与选项C对应，属于同义替换，他们已经无法摆脱毒品，故答案选C。