Question: when did nigeria become a member of opec
[{'answer': '1971', 'provenance': [{'wikipedia_id': '166346', 'title': 'OPEC', 'start_paragraph_id': 43, 'start_character': 226, 'end_paragraph_id': 43, 'end_character': 230, 'bleu_score': 1.0, 'section': 'Section::::History and impact.:1960–1975 founding and expansion.\n'}]}]