Question: when did the great wall of china started building
[{'answer': '221 -- 206 BC'}, {'provenance': [{'wikipedia_id': '34476548', 'title': 'History of the Great Wall of China', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 454, 'bleu_score': 0.7789407038334321, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9891918239501436836}, 'section': 'Section::::Abstract.'}]}]