Question: when does the champions league group stages start
[{'answer': '12 September', 'provenance': [{'wikipedia_id': '54325979', 'title': '2017–18 UEFA Champions League group stage', 'start_paragraph_id': 1, 'start_character': 55, 'end_paragraph_id': 1, 'end_character': 67, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '54325979', 'title': '2017–18 UEFA Champions League group stage', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 228, 'bleu_score': 0.6788575709004121, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 1083220305406822741}, 'section': 'Section::::Abstract.'}]}]