Question: when did us involvement in the vietnam war end
[{'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.'}]}]