Head injury is the most serious type of injury sustained in motorcycle accidents. The average cost to taxpayers for medical care for nonhelmeted motorcycle-accident victims is twice that for their helmeted counterparts. Jurisdictions that have enacted motorcycle-helmet laws have reduced the incidence and severity of accident-related head injuries, thereby reducing the cost to taxpayers. Ttherefore, to achieve similar cost reductions, other jurisdictions should enact motorcycle-helmet laws. For the same reason jurisdictions should also require helmets for horseback riders, since horseback-riding accidents are even more likely to cause serious head injury than motorcycle accidents are. Which one of the following is an assumption upon which the author's conclusion concerning helmets for horseback riders depends? ["When deciding whether to enact helmet laws for motorcyclists and horseback riders, the jurisdiction's primary concern is the safety of its citizens.", 'The medical costs associated with treating head injuries are higher than those for other types of injury.', 'The higher rate of serious head injury suffered by victims of horseback-riding accidents is due to the difference in size between horses and motorcycles.', 'Medical care for victims of horseback-riding accidents is a financial drain on tax funds.']