Geothermal power plants produce power using heat from underground reservoirs of hot water or steam heated by the surrounding rock. In the limited areas of the world where such underground hot water and steam can currently be reached by drilling, geothermal power plants produce power more economically than conventional, fossil fuel power plants. However, advocates contend that in the near future economical power from geothermal power plants will be available in most areas. Which one of the following, if true, most helps to justify the advocates' contention? ['Conventional power plants, unlike geothermal power plants, release large amounts of pollutants into the air.', 'The high start-up costs of geothermal power plants discourages their construction even in locations where they are more economical than conventional power plants in the long run.', 'Advanced drilling technology is being developed that will soon make it both feasible and economical to drill wells many times deeper than it is currently possible to drill.', 'Recent research has led to discoveries that could significantly lower production costs for nearly all types of power plants.']