Question: who is the youngest elected president of usa
[{'answer': 'John F. Kennedy', 'provenance': [{'wikipedia_id': '11262088', 'title': 'List of presidents of the United States by age', 'start_paragraph_id': 3, 'start_character': 401, 'end_paragraph_id': 3, 'end_character': 416, 'bleu_score': 1.0, 'section': 'Section::::Age of presidents.\n'}]}]