填空题

• A. 10 percent to 25 percent for the proportion of workers who use drugs occasionally on the job
• B. determine exactly when simple drag use becomes abuse
• C. the 1920s until the 1960s
• D. the 1960s until the 1970s
• E. signals passing through the nervous system
• F. describe the phenomenon of drug use

It is not always easy to ______.