填空题

下面的短文后有2项测试任务：(1)第23～26题要求从所给的6个选项中为第3～6段每段选择1个最佳标题：(2)第27～30题要求从所给的6个选项中为每个句子确定1个最佳选项。
Drug Abuse
1 The term "drug abuse" most often refers to the use of a drug with such frequency that it causes physical or mental harm to the user or impairs social functioning. Although the term seems to imply that users abuse the drugs they take, in fact, it is themselves or others they abuse by using drugs.
2 Pharmacologists, who study the effects of drugs, classify psychoactive drugs according to what they do to those who take them. Drugs that speed up signals passing through the nervous system, which is made up of the brain and spinal cord, and produce alertness and arousal in higher doses , excitability, and inhibit fatigue and sleep, are called stimulants. Drugs that retard, slow down, or depress signals passing through the central nervous system and produce relaxation, a lowering of anxiety, and, at higher doses, drowsiness and sleep, are called depressants. One distinct kind of depressants are those which dull the mind’s perception of pain and in medicine are used as painkillers, or analgesics. These drugs called narcotics.
3 It is not always easy to determine exactly when simple drug use becomes abuse. Thus it is far easier to study who uses illegal psychoactive drugs than it is to study who abuses them. When researchers describe patterns of drug abuse, then, they usually describe the more general phenomenon of drug use, whether it leads to abuse or not.
4 Drinking on the job is a social and economic problem with a long history. With the growing popularity of illegal drugs in the 1960s and 1970s, it was to be expected that their use in the workplace would emerge as a major issue by the 1980s. Estimates of employee drug use vary greatly, ranging from 10 percent to 25 percent for the proportion of workers who use drugs occasionally on the job.
5 From the 1920s until the 1960s, treatment of drug abuse in the United States was practically nonexistent. During this period many officials did not believe that treatment was effective or necessary. Drug abusers and sellers were simply arrested and imprisoned, thereby discouraging use. The dramatic explosion in the use and abuse of a wide range of different drugs during the 1960s demonstrated the weakness of this theory. As a result, two treatment programs were developed during the 1960s.
A. Patterns of drug abuse
B. Treatment
C. Drug testing in the workplace
D. Classification of psychoactive drugs
E. Definition Paragraph 5 ______.