GMAT考试写作指导：Argument范文四八

　　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 authors 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

　　leaves open the possibility that factors other than the one cited are responsible for the

　　increase in cocaine use.

　　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.

　　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.