A certain auto repair shop services a particular make of car in the downtown region of a particular city. Three years ago, 4% of all repairs at this shop were not successful --- the mechanics were not able to restore the car to a drivable state. This year, only 2% of all repairs were not successful. Clearly, the mechanics training course, given over a year ago, has considerably increased the skill of mechanics at this particular shop, so that fewer cars are beyond their ability to repair. All of the following statements, if true, weaken the argument, EXCEPT: ['The manufacturer of this particular make of car recalled a few models due to defects that potentially jeopardized the engine.', 'The total number of cars served per year by this shop has increased by 35% over the past three years.', 'Three years ago, statewide smog regulations had the effect of removing the oldest and most unreliable vehicles from the road.', 'A new shop uptown specializes in repairing cars of this make more than six years old, and most of these older cars are taken to this shop.']