Question: when did the vietnam war end for the us
[{'answer': '15 August 1973', 'provenance': [{'wikipedia_id': '32611', 'title': 'Vietnam War', 'start_paragraph_id': 4, 'start_character': 678, 'end_paragraph_id': 4, 'end_character': 692, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '32611', 'title': 'Vietnam War', 'start_paragraph_id': 5, 'start_character': 0, 'end_paragraph_id': 5, 'end_character': 349, 'bleu_score': 0.515343603785656, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 12680022634771195623}, 'section': 'Section::::Abstract.'}]}]