In the United States, average fuel efficiency of newly manufactured domestic cars, although remaining worse than that of newly manufactured imported cars, substantially improved between 1983 and 1988. Average fuel efficiency of new domestic cars has not improved since, but the difference in average fuel efficiencies of new domestic cars and new imported cars has steadily decreased. If the statements above are true, which of the following must also be true on the basis of them? ['Average fuel efficiency of imported cars manufactured in 1983 was better than that of imported cars manufactured in 1988.', 'Average fuel efficiency of newly manufactured imported cars has steadily improved since 1983.', 'Average fuel efficiency of newly manufactured domestic cars has steadily worsened since 1988.', 'Average fuel efficiency of newly manufactured imported cars has steadily worsened since 1988.']