Plastic has been replacing aluminum in a widening array of military equipment because it offers equal protection at less weight. A plastic-armored vehicle is, however, twice as costly to make as an aluminum-armored vehicle. Ttherefore, the replacement of aluminum-armored vehicles by plastic-armored vehicles that offer equal protection is not yet advisable from a financial point of view. Which of the following, if true, casts most doubt on the conclusion above? ['Although the cost of plastic is expected to remain stable, the growing practice of recycling metals will probably drive down the cost of metals.', 'The cost of repairing body damage on plastic-armored vehicles would exceed what the military currently spends to repair body damage on aluminum-armored vehicles.', 'A plastic-armored vehicle could be operated at one-third the current cost of operating an aluminum-armored vehicle.', 'The aluminum-armored vehicle is the least costly to maintain of all the metal-armored vehicles currently used by the military.']