Question: when was the first king arthur story written
[{'answer': 'c. 1138'}, {'provenance': [{'wikipedia_id': '16808', 'title': 'King Arthur', 'start_paragraph_id': 25, 'start_character': 0, 'end_paragraph_id': 25, 'end_character': 1795, 'bleu_score': 0.8857402531729631, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 2919200056367811448}, 'section': 'Section::::Medieval literary traditions.:Geoffrey of Monmouth.\n'}]}]