GMAT Sentence Correction (SC) | Re: In studying pre-industrial Europe, these historians have focused

In studying pre-industrial Europe, these historians have focused on only the 2 or 3 percent of the population that comprised the political and social elite, but they have generally overlooked the rest, thereby failing to understand European civilization as a whole.

(A) In studying pre-industrial Europe, these historians have focused on
(correct, historians is the subject)
(B) Studies of pre-industrial Europe by these historians have been focused on
(passive voice error)
(C) Studying pre-industrial
...

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