In 1727, dramatist Lewis Theobald presented a new play, Double Falsehood, at a London theater. Theobald claimed that his drama was based on a little-known play by William Shakespeare, Cardenio. Many, including poet Alexander Pope, were ________ historians have determined that Shakespeare's company did perform a play called Cardenio in 1613.

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