Question: when did the air force leave the army
[{'answer': '18 September 1947', 'provenance': [{'wikipedia_id': '9895061', 'title': 'History of the United States Air Force', 'start_paragraph_id': 1, 'start_character': 66, 'end_paragraph_id': 1, 'end_character': 83, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '9895061', 'title': 'History of the United States Air Force', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 848, 'bleu_score': 0.9853469949554711, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9932963362460279658}, 'section': 'Section::::Abstract.'}]}]