SAT Question a466679a: Boundaries answer and explanation

English - Boundaries - No Punctuation Within Clause
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In 1976, the Inuit rock group Sikumiut recorded the album People of the Ice. Though only their first record, it shows a band already skilled at the difficult task of making music that sounds easy and fun. On songs like “Utirumavunga,” Lucassie Koperqualuk’s guitar riffs effortlessly ______ Charlie Adams’s delightfully catchy vocal melodies.
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Difficulty:Easy
Which choice completes the text so that it conforms to the conventions of Standard English?