Transportation expenses accounted for a large portion of the total dollar amount spent on trips for pleasure by residents of the United States in 1997, and about half of the total dollar amount spent on the transportation was airfare. However, the large majority of United States residents who took trips for pleasure in 1997 did not travel by airplane but used other means of transportation. If the statements above are true, which of the following must also be true about United States residents who took trips for pleasure in 1997? ['Most of those who traveled by airplane did so because the airfare to their destination was lower than the cost of other available means of transportation.', 'Per mile traveled, those who traveled by airplane tended to spend more on transportation to their destination than did those who used other means of transportation.', 'Most of those who traveled by airplane did so because other means of transportation to their destinations were unavailable.', 'Overall, people who did not travel by airplane had lower average transportation expenses than people who did.']