Question: when did north vietnam and south vietnam unite
[{'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.'}]}]