Question: when did chicago bulls win their first nba championship
[{'answer': '1991', 'provenance': [{'wikipedia_id': '72866', 'title': 'Chicago Bulls', 'start_paragraph_id': 2, 'start_character': 218, 'end_paragraph_id': 2, 'end_character': 222, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]