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Question 168·200 Super-Hard SAT Reading Questions·Standard English Conventions

Although the surviving copies of a fourteenth-century navigational handbook differ in many details, the sequence of marginal symbols in several of them—marks that historian Amara Okafor argues ______ not routine abbreviations made by copyists but an indexing system devised by working sailors—has remained remarkably consistent.

Which choice completes the text so that it conforms to the conventions of Standard English?