Nothing.
Without an answer key, a student studying alone has no way to verify their historical reasoning. A 2016 student might correctly identify that the "Gilded Age" involved industrial trusts, but they might miss the nuance of a question asking for the primary cause of labor unrest. The answer key provides not just a letter (A, B, C, D) but often rationales—explanations for why an answer is correct and why the others are wrong. amsco 2016 answer key