Question: what is the longest river in poland called
[{'answer': 'Vistula', 'provenance': [{'wikipedia_id': '395044', 'title': 'List of rivers of Poland', 'start_paragraph_id': 27, 'start_character': 12, 'end_paragraph_id': 27, 'end_character': 19, 'bleu_score': 1.0, 'section': 'Section::::River system.\n'}]}]