Question: when did the earth's human population reach six billion
[{'answer': 'October 12 , 1999', 'provenance': [{'wikipedia_id': '19017269', 'title': 'World population', 'start_paragraph_id': 24, 'start_character': 426, 'end_paragraph_id': 24, 'end_character': 443, 'bleu_score': 1.0, 'section': 'Section::::History.:Milestones by the billions.\n'}]}, {'provenance': [{'wikipedia_id': '19017269', 'title': 'World population', 'start_paragraph_id': 24, 'start_character': 0, 'end_paragraph_id': 24, 'end_character': 544, 'bleu_score': 0.9792717993254391, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5813451217134677619}, 'section': 'Section::::History.:Milestones by the billions.\n'}]}]