Question: when did the national drinking age become 21
[{'answer': 'mid-1988', 'provenance': [{'wikipedia_id': '19197423', 'title': 'U.S. history of alcohol minimum purchase age by state', 'start_paragraph_id': 4, 'start_character': 364, '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': 8055225052508844345}, 'section': 'Section::::Abstract.'}]}]