Engineer: Thermophotovoltaic generators are devices that convert heat into electricity. The process of manufacturing steel produces huge amounts of heat that currently go to waste. So if steel-manufacturing plants could feed the heat they produce into thermophotovoltaic generators, they would greatly reduce their electric bills, thereby saving money. Which one of the following is an assumption on which the engineer's argument depends? ['There is no other means of utilizing the heat produced by the steel-manufacturing process that would be more cost effective than installing thermophotovoltaic generators.', 'Using current technology, it would be possible for steel-manufacturing plants to feed the heat they produce into thermophotovoltaic generators in such a way that those generators could convert at least some of that heat into electricity.', 'The amount steel-manufacturing plants would save on their electric bills by feeding heat into thermophotovoltaic generators would be sufficient to cover the cost of purchasing and installing those generators.', 'At least some steel-manufacturing plants rely on electricity as their primary source of energy in the steel-manufacturing process.']