GMAT Sentence Correction (SC) | Re: The original Ancient Greek gymnasiums were incredibly advanced and had

daagh chetan2u Engr2012

Hi, Could you please explain the usage of dependent clause - "long before teachers such as Plato and Aristotle had begun their work" in this sentence? I understand the verb error here, but how is this dependent clause modifying the action of Verb-ING modifier?

Thanks a lot!

YT
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