In the past, when there was no highway speed limit, the highway accident rate increased yearly, peaking a decade ago. At that time, the speed limit on highways was set at 90 kilometers per hour (kph) (55 miles per hour). Every year since the introduction of the highway speed limit, the highway accident rate has been at least 15 percent lower than that of its peak rate. Thus, setting the highway speed limit at 90 kph (55 mph) has reduced the highway accident rate by at least 15 percent. Which one of the following, if true, most seriously weakens the argument? ['Thanks to changes in automobile design in the past ten years, drivers are better able to maintain control of their cars in dangerous situations.', 'It was not until shortly after the introduction of the highway speed limit that most cars were equipped with features such as seat belts and airbags designed to prevent harm to passengers.', 'In the years prior to the introduction of the highway speed limit, many cars could go faster than 90 kph (55 mph).', 'Ten years ago, at least 95 percent of all automobile accidents in the area occurred on roads with a speed limit of under 80 kph (50 mph).']