SAT Question e44db0a0: Form Structure And Sense answer and explanation

English - Form Structure And Sense - Subject Verb Agreement
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Because a cycle of lunar phases ______ 29.5 days to complete, it’s possible to observe two full moons in a single month, one at the beginning and one at the end.
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Difficulty:Medium
Which choice completes the text so that it conforms to the conventions of Standard English?