Question: when was the constitution of united states signed
[{'answer': 'September 17 , 1787', 'provenance': [{'wikipedia_id': '51107255', 'title': 'Signing of the United States Constitution', 'start_paragraph_id': 18, 'start_character': 48, 'end_paragraph_id': 18, 'end_character': 74, 'bleu_score': 1.0, 'section': 'Section::::Accompanying documents.\n'}]}, {'provenance': [{'wikipedia_id': '51107255', 'title': 'Signing of the United States Constitution', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 918, 'bleu_score': 0.9851290761715561, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5121553910391475149}, 'section': 'Section::::Abstract.'}]}]