Question: when was the last census held in nigeria
[{'answer': '2006', 'provenance': [{'wikipedia_id': '37641052', 'title': 'Population and housing censuses by country', 'start_paragraph_id': 17, 'start_character': 115, 'end_paragraph_id': 17, 'end_character': 119, 'bleu_score': 1.0, 'section': 'Section::::Africa.:Burkina Faso.\n'}]}, {'provenance': [{'wikipedia_id': '37641052', 'title': 'Population and housing censuses by country', 'start_paragraph_id': 36, 'start_character': 0, 'end_paragraph_id': 36, 'end_character': 400, 'bleu_score': 0.9347935651229401, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 2746428839550952549}, 'section': 'Section::::Africa.:Nigeria.\n'}]}]