Question: when did the world population reach 6 billion
[{'answer': '1999', 'provenance': [{'wikipedia_id': '33666830', 'title': 'World population milestones', 'start_paragraph_id': 2, 'start_character': 360, 'end_paragraph_id': 2, 'end_character': 364, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]