A、On short trips. B、On long trips. C、When flying over cities. D、When flying at high altitude. A

Although I think the United States generally has an excellent system of transporting people between cities that .are only a few hundred miles apart, a person commuting between Detroit and Chicago, or between san Francisco and Los Angeles, so-called strip cities, may spend only a relatively short time in the air while spending several hours getting to and from the airport. This situation makes flying almost as time-consuming as driving. Moreover, airplanes use a lot of their fuel just getting into the air. They simply are not fuel-efficient on short trips.
    High-speed trains may be an answer. One fairly new proposal for such a train is for something called a "maglev", meaning a magnetically-levitated train.
    Maglevs will not actually ride on the tracks, but will fly above tracks that are magnetically activated. This will save wear and tear on the tracks. These trains will be able to go faster than one hundred fifty miles per hour. At that speed, conventional trains have trouble staying on the tracks. As you can see, maglevs offer exciting possibilities for the future.