Question: when was the united states post office established
[{'answer': 'July 1 , 1971', 'provenance': [{'wikipedia_id': '50591', 'title': 'United States Postal Service', 'start_paragraph_id': 6, 'start_character': 840, 'end_paragraph_id': 6, 'end_character': 853, 'bleu_score': 1.0, 'section': 'Section::::History.\n'}]}]