Aroca City currently funds its public schools through taxes on property. In place of this system, the city plans to introduce a sales tax of 3 percent on all retail sales in the city. Critics protest that 3 percent of current retail sales falls short of the amount raised for schools by property taxes. The critics are correct on this point. Nevertheless, implementing the plan will probably not reduce the money going to Aroca's schools. Several large retailers have selected Aroca City as the site for huge new stores, and these are certain to draw large numbers of shoppers from neighboring municipalities, where sales are taxed at rates of 6 percent and more. In consequence, retail sales in Aroca City are bound to increase substantially.
In the argument given, the two portions in boldface play which of the following roles?
【选项】The first presents a plan that the argument concludes is unlikely to achieve its goal; the second presents evidence in support of that conclusion.