In the nation of Partoria, large trucks currently have a much higher rate of traffic accidents per mile driven than other vehicles do. However, the very largest trucks-those with three trailers-had less than a third of the accident rate of single- and double-trailer trucks. Clearly, ttherefore, one way for Partoria to reduce the number of traffic accidents would be to require shippers to increase their use of triple-trailer trucks. Which of the following, if true, most seriously weakens the argument? ['In Partoria, the safety record of the trucking industry as a whole has improved slightly over the past ten years.', 'No matter what changes Partoria makes in the regulation of trucking, it will have to keep some smaller roads off-limits to all large trucks.', "Increased use of triple-trailer trucks would mean that large trucks would account for a smaller proportion of all miles driven on Partoria's roads than they currently do.", 'Partorian trucking companies currently use triple-trailer trucks only for long trips using major highways, which is the safest kind of trip for large trucks.']