In Western economies, more energy is used to operate buildings than to operate transportation. Much of the decline in energy consumption since the oil crisis of 1973 is due to more efficient use of energy in homes and offices. New building technologies, which make lighting, heating, and ventilation systems more efficient, have cut billions of dollars from energy bills in the West. Since energy savings from these efficiencies save several billion dollars per year today, we can conclude that 50 to 100 years from now they will save more than $200 billion per year (calculated in current dollars). On which one of the following assumptions does the argument rely? ['Technology used to make buildings energy efficient will not become prohibitively expensive over the next century.', 'Energy-efficient technologies based on new scientific principles will be introduced in the next 50 to 100 years.', 'Energy bills in the West will be $200 billion lower in the next 50 to 100 years.', 'Another oil crisis will occur in the next 50 to 100 years.']