Question: when was the big walker mountain tunnel built
[{'answer': '1972', 'provenance': [{'wikipedia_id': '1833481', 'title': 'Big Walker Mountain Tunnel', 'start_paragraph_id': 3, 'start_character': 52, 'end_paragraph_id': 3, 'end_character': 56, 'bleu_score': 1.0, 'section': 'Section::::History.\n'}]}, {'provenance': [{'wikipedia_id': '1833481', 'title': 'Big Walker Mountain Tunnel', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 331, 'bleu_score': 0.8627328633221021, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11733074307585407808}, 'section': 'Section::::History.\n'}]}]