Can someone explain to me why the answer isn't D?

My thinking was that D has to be assumed because if helmet wouldn't prevent the fatality then there would be no point in requiring the helmet since they would just be dead anyway? Morbid thinking but...

Stimulus:

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. Therefore, 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 depend?

(A) Medical care for victims of horseback-riding accidents is financial drain on tax funds.

(B) 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.

(C) The medical costs associated with treating head injuries are higher than those for other types of injury.

(D) Most fatalities resulting from horseback-riding and motorcycle accidents could have been prevented if the victims had been wearing helmets.

(E) When deciding whether to enact helmet laws for motorcyclists and horseback riders, the jurisdiction's primary concerns is the safety of its citizens.

Admin note: edited title; please use the format of "PT#.S#.Q# - [brief description]"

Anyone have any tips on how to tackle these questions? Usually goes: which one of the following would be most useful in evaluating the argument made. I've been trying to identify an assumption and pick the answer choice that is relevant to that assumption but it's working 50/50.