Question: when did the us involvement in vietnam end
[{'answer': '30 April 1975', 'provenance': [{'wikipedia_id': '32611', 'title': 'Vietnam War', 'start_paragraph_id': 1, 'start_character': 236, 'end_paragraph_id': 1, 'end_character': 249, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '32611', 'title': 'Vietnam War', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 733, 'bleu_score': 0.63829109880321, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 6228934248661479119}, 'section': 'Section::::Abstract.'}]}]