In the nation of Partoria, large trucks currently account for 6 percent of miles driven on Partoria' s roads but are involved in 12 percent of all highway fatalities. 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 highway deaths would be to require shippers to increase their use of triple-trailer trucks. Which of the following, if true, most seriously weakens the argument? ['Very few fatal collisions involving trucks in Partoria are collisions between two trucks.', '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.', 'Partorian trucking companies have so far used triple-trailer trucks on lightly traveled sections of major highways only.', 'In Partoria, the safety record of the trucking industry as a whole has improved slightly over the past ten years.']