The older a car is, the more likely it is to be discarded for scrap rather than repaired when major engine work is needed to keep the car in operation. Government motor vehicle regulations generally require all cars, whether old or new, to be inspected for emission levels prior to registration. Many of the repairs that were performed on the cars that the National Motor Company built in the 1960s could have been avoided if periodic routine maintenance had been performed. The cars that the National Motor Company built in the 1970s incorporated simplified engine designs that made the engines less complicated than those of earlier models.
In 1990 major engine repairs were performed on 10 percent of the cars that had been built by the National Motor Company in the 1970s and that were still registered. However, the corresponding figure for the cars that the National Motor Company had manufactured in the 1960s was only five percent.
Which one of the following, if true, most helps to explain the discrepancy?