Question: what is the most posterior structure of the atlas c1 vertebra
[{'answer': 'posterior arch', 'provenance': [{'wikipedia_id': '47430', 'title': 'Atlas (anatomy)', 'start_paragraph_id': 5, 'start_character': 104, 'end_paragraph_id': 5, 'end_character': 118, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '47430', 'title': 'Atlas (anatomy)', 'start_paragraph_id': 5, 'start_character': 0, 'end_paragraph_id': 5, 'end_character': 141, 'bleu_score': 0.929712191543383, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8261003256990501688}, 'section': 'Section::::Abstract.'}]}]