Question: when did the star spangled banner offically become the national anthem
[{'answer': '1931', 'provenance': [{'wikipedia_id': '8872619', 'title': 'Timeline of the flag of the United States', 'start_paragraph_id': 7, 'start_character': 170, 'end_paragraph_id': 7, 'end_character': 174, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '8872619', 'title': 'Timeline of the flag of the United States', 'start_paragraph_id': 7, 'start_character': 0, 'end_paragraph_id': 7, 'end_character': 514, 'bleu_score': 0.7699678850528391, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13304044267169266281}, 'section': 'Section::::Abstract.'}]}]