Question: what unit of geologic time is the greatest span of time
[{'answer': 'eons', 'provenance': [{'wikipedia_id': '12967', 'title': 'Geologic time scale', 'start_paragraph_id': 3, 'start_character': 42, 'end_paragraph_id': 3, 'end_character': 46, 'bleu_score': 1.0, 'section': 'Section::::Terminology.\n'}]}]