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?