SAT Question 2c9c6ca9: Boundaries answer and explanation

English - Boundaries - Semicolon Between Independent Clauses
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The term “retroflex” derives from Latin and means “bent back,” an apt descriptor for the branch of consonants—retroflex consonants—pronounced with the tongue curling up and back in the mouth. In many languages, including English, these consonants are ______ in some dialects of Mandarin, however, four such consonants (“ch,” “sh,” “zh,” and “r”) are relatively common.
30
Difficulty:Hard
Which choice completes the text so that it conforms to the conventions of Standard English?