In the past the country of Siduria has relied heavily on imported oil. Siduria recently implemented a program to convert heating systems from oil to natural gas. Siduria already produces more natural gas each year than it burns, and oil production in Sidurian oil fields is increasing at a steady pace. If these trends in fuel production and usage continue, ttherefore, Sidurian reliance on foreign sources for fuel should decline soon. Which of the following is an assumption on which the argument depends? ['Buildings cannot be heated by solar energy rather than by oil or natural gas.', 'No fuel other than natural gas is expected to be used as a replacement for oil in Siduria.', 'In Siduria the rate of fuel consumption is rising no more quickly than the rate of fuel production.', 'All new homes that are being built will have natural-gas-burning heating systems.']