Question: when was the last time the sahara was green
[{'answer': '3900 BCE', 'provenance': [{'wikipedia_id': '12746283', 'title': 'Neolithic Subpluvial', 'start_paragraph_id': 4, 'start_character': 158, 'end_paragraph_id': 4, 'end_character': 166, 'bleu_score': 1.0, 'section': 'Section::::Date ranges.\n'}]}, {'provenance': [{'wikipedia_id': '12746283', 'title': 'Neolithic Subpluvial', 'start_paragraph_id': 4, 'start_character': 0, 'end_paragraph_id': 4, 'end_character': 382, 'bleu_score': 0.923768200931469, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11695774329477044871}, 'section': 'Section::::Date ranges.\n'}]}]