They were transmitted orally from generation to generation and were not set down in writing until centuries after they were first sung.
它们只是通过一代代人口头流传，直到几个世纪后才有文字形式。
They were transmitted orally from generation to generation and were not set down in writing until centuries after they were first sung.
它们只是通过一代代人口头流传，直到几个世纪后才有文字形式。
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