Question: when was 50 naira currency note introduced into nigeria
[{'answer': '1991', 'provenance': [{'wikipedia_id': '178686', 'title': 'Nigerian naira', 'start_paragraph_id': 8, 'start_character': 202, 'end_paragraph_id': 8, 'end_character': 206, 'bleu_score': 1.0, 'section': 'Section::::History.:Coins.\n'}]}, {'provenance': [{'wikipedia_id': '178686', 'title': 'Nigerian naira', 'start_paragraph_id': 18, 'start_character': 0, 'end_paragraph_id': 18, 'end_character': 284, 'bleu_score': 0.965203995518735, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8742823257063945030}, 'section': 'Section::::History.:Banknotes.\n'}]}]