Technological improvements and reduced equipment costs have made converting solar energy directly into electricity far more cost-efficient in the last decade. However, the threshold of economic viability for solar power (that is, the price per barrel to which oil would have to rise in order for new solar power plants to be more economical than new oil-fired power plants) is unchanged at thirty-five dollars. Which of the following, if true, does most to help explain why the increased cost-efficiency of solar power has not decreased its threshold of economic viability? ['Technological changes have increased the efficiency of oil-fired power plants.', 'Most electricity is generated by coal-fired or nuclear, rather than oil-fired, power plants.', 'The cost of oil has fallen dramatically.', 'When the price of oil increases, reserves of oil not previously worth exploiting become economically viable.']