Older United States automobiles have been identified as contributing disproportionately to global air pollution. The requirement in many jurisdictions that automobiles pass emission-control inspections has had the effect of taking many such automobiles out of service in the United States, as they fail inspection and their owners opt to buy newer automobiles. Thus the burden of pollution such older United States automobiles contribute to the global atmosphere will be gradually reduced over the next decade. Which one of the following, if true, most seriously weakens the argument? ['There is a thriving market for used older United States automobiles that are exported to regions that have no emission-control regulations.', 'When automobiles that are now new become older, they will, because of a design change, cause less air pollution than older automobiles do now.', 'Even if all the older automobiles in the United States were retired from service, air pollution from United States automobiles could still increase if the total number of automobiles in use should increase significantly.', 'It is impossible to separate the air of one country or jurisdiction from that of others, since air currents circle the globe.']