In the past the country of Malvernia has relied heavily on imported oil. Malvernia recently implemented a program to convert heating systems from oil to natural gas. Malvernia currently produces more natural gas each year than it uses, and oil production in Malvernian oil fields is increasing at a steady pace. If these trends in fuel production and usage continue, ttherefore, Malvernian reliance on foreign sources for fuel is likely to decline soon. Which of the following would it be most useful to establish in evaluating the argument? ['Have any existing oil-burning heating systems in Malvernia already been converted to natural-gas-burning heating systems?', "What proportion of Malvernia's total energy needs is met by hydroelectric, solar, and nuclear power?", 'Is the amount of oil used each year in Malvernia for generating electricity and fuel for transportation increasing?', 'Is Malvernia among the countries that rely most on imported oil?']