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22.       
　　The conclusion in this argument is that increased vigilance by drug enforcement authorities has resulted in an increase in the illegal use of cocaine. The author reaches this conclusion on the grounds that drug traffickers have responded to increased enforcement efforts by switching from bulkier and riskier drugs to cocaine. Presumably, the author's reasoning is that the increased enforcement efforts inadvertently brought about an increase in the supply of cocaine which, in turn,brought about the observed increase in the illegal use of cocaine. This line of reasoning is problematic in two important respects.

      In the first place, the author has engaged in "after this, therefore because of this" reasoning. The only reason offered for believing that the increased vigilance caused the increase in cocaine use is the fact that the former preceded the latter. No additional evidence linking the two events is offered in the argument, thus leaving open the possibility that the two events are not causally related but merely correlated. This in turn eaves open the possibility that factors other than the one cited are responsible for the increase in cocaine use.来源：考试大

      In the second place, the author assumes that an increase in the supply of cocaine is sufficient to bring about an increase in its use. While this is a tempting assumption, it is a problematic one. The presumption required to substantiate this view is that drug users are not particular about which drugs they use, so that if marijuana and heroin are not available, they will switch to whatever drug is available—cocaine in this case. The assumption does not seem reasonable on its face. Marijuana, heroin, and cocaine are not alike in their effects on users; nor are they alike in the manner in which they are ingested or in their addictive properties. The view that drug users' choice of drugs is simply a function of supply overlooks these important differences.来源：考试大

      In conclusion, the author has failed to establish a causal link between increased enforcement efforts and the observed increase in illegal cocaine use. While the enforcement activities may have been a contributing factor, to show a clear causal connection the author must examine and rule out various other factors.