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水平法和累计法的选择依据是计划指标。 ()
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经过三年的国民经济恢复,自1953年起,我国进入第一个五年计划时期,它是第一次()。
听力原文： Although I think the United States generally has an excellent system of transportation, I don' t think that it does a good job 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. Magievs 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 150 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.
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设有下列二叉树：
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