Question: when did they stop calling russia the soviet union
[{'answer': '1991', 'provenance': [{'wikipedia_id': '26779', 'title': 'Soviet Union', 'start_paragraph_id': 1, 'start_character': 153, 'end_paragraph_id': 1, 'end_character': 157, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]