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翻译(英译汉)
51.The historian Frederick J. Tuner wrote in the 1890's that the agrarian(农民)discontent(不满)that had been developing steadily in the United States since about 1870 had been precipitated(加速)by the closing of the internal frontier-that is, the depletion(枯竭)of available new land needed for further expansion of the American farming system.
答案：历史学家Frederick J Tuner在19世纪90年代写道：自1870年以来美国农民的不满一直在缓慢增长，直到1870年关闭了内部前沿以后就被加速了——就是说，用于扩张农业系统的新土地资源已经枯竭了。
