Question: when did the world population reach 5.9 billion
[{'answer': 'six billion in 1999', 'provenance': [{'wikipedia_id': '19017269', 'title': 'World population', 'start_paragraph_id': 22, 'start_character': 298, 'end_paragraph_id': 22, 'end_character': 317, 'bleu_score': 1.0, 'section': 'Section::::History.:Milestones by the billions.\n'}]}, {'provenance': [{'wikipedia_id': '19017269', 'title': 'World population', 'start_paragraph_id': 22, 'start_character': 0, 'end_paragraph_id': 22, 'end_character': 500, 'bleu_score': 0.9766041382465441, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13298109742690403693}, 'section': 'Section::::History.:Milestones by the billions.\n'}]}]