Question: when did the last american combat troops leave vietnam
[{'answer': 'end of March 1973'}, {'provenance': [{'wikipedia_id': '32611', 'title': 'Vietnam War', 'start_paragraph_id': 114, 'start_character': 0, 'end_paragraph_id': 114, 'end_character': 409, 'bleu_score': 0.5406626240646161, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11076328729691666900}, 'section': 'Section::::Nixon Doctrine and Vietnamization, 1969–72.:Easter Offensive and Paris Peace Accords, 1972.\n'}]}]