Question: when did the world population reach 2 billion
[{'answer': '1927', 'provenance': [{'wikipedia_id': '19017269', 'title': 'World population', 'start_paragraph_id': 22, 'start_character': 148, 'end_paragraph_id': 22, 'end_character': 152, 'bleu_score': 1.0, 'section': 'Section::::History.:Milestones by the billions.\n'}]}]