The central idea of the article, The brief history of summer vacation by Jackson Murphy is B. Low-paid, seasonal workers were the only group who didn't benefit from summer vacations.
This refers to the main message that an author wants to convey to his audience and contains a moral lesson or enduring theme.
Hence, we can see that from the complete text, there is the narration of the culture of summer vacations and how it was advertised as a relaxation period and the low-paid workers did not benefit as they traveled for work, not leisure.
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