Question: what is the cost of a first-class stamp in the united states
[{'answer': '. 50', 'provenance': [{'wikipedia_id': '1653050', 'title': 'History of United States postage rates', 'start_paragraph_id': 8, 'start_character': 73, 'end_paragraph_id': 8, 'end_character': 76, 'bleu_score': 1.0, 'section': 'Section::::Historical trend.\n'}]}]