Question: when did the legal drinking age changed to 21
[{'answer': 'October 1986', 'provenance': [{'wikipedia_id': '19197423', 'title': 'U.S. history of alcohol minimum purchase age by state', 'start_paragraph_id': 4, 'start_character': 304, 'end_paragraph_id': 4, 'end_character': 316, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'By mid-1988', 'provenance': [{'wikipedia_id': '19197423', 'title': 'U.S. history of alcohol minimum purchase age by state', 'start_paragraph_id': 4, 'start_character': 361, 'end_paragraph_id': 4, 'end_character': 372, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '19197423', 'title': 'U.S. history of alcohol minimum purchase age by state', 'start_paragraph_id': 4, 'start_character': 0, 'end_paragraph_id': 4, 'end_character': 1108, 'bleu_score': 0.9765707625998521, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 3962062183099180607}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '19197423', 'title': 'U.S. history of alcohol minimum purchase age by state', 'section': 'Section::::Abstract.', 'start_paragraph_id': 4, 'end_paragraph_id': 4, 'meta': {'evidence_span': ['which required states to raise their ages for purchase and public possession to 21 by October 1986', 'In 1984, Congress passed the National Minimum Drinking Age Act, which required states to raise their ages for purchase and public possession to 21 by October 1986 or lose 10% of their federal highway funds.', 'mid-1988']}}]}]