SAT Question 983d33fa: Form Structure And Sense answer and explanation

English - Form Structure And Sense - Finite And Nonfinite Verb Forms
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In 1637, the price of tulips skyrocketed in Amsterdam, with single bulbs of rare varieties selling for up to the equivalent of $200,000 in today’s US dollars. Some historians ______ that this “tulip mania” was the first historical instance of an asset bubble, which occurs when investors drive prices to highs not supported by actual demand.
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Difficulty:Easy
Which choice completes the text so that it conforms to the conventions of Standard English?