To combat persistent counterfeiting, Lackland's currency was redesigned to include images that cannot be convincingly duplicated by the means used to successfully counterfeit the old bills. Last year, after the old currency was replaced, many crude counterfeits of the new bills were detected. But now it has been several months since any counterfeit currency has been found. Therefore, it is clear that counterfeiters have largely abandoned their attempt to reproduce the new currency, always assuming that _______.

Which of the following most logically completes the argument?