SAT Question 960dec02: Boundaries answer and explanation

English - Boundaries - No Punctuation Within Clause
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A recent study tracked the number of bee species present in twenty-seven New York apple orchards over a ten-year period. ______ found that when wild growth near an orchard was cleared, the number of different bee species visiting the orchard decreased.
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Difficulty:Hard
Which choice completes the text so that it conforms to the conventions of Standard English?