Question: who has the most subscribers on youtube in the us
[{'answer': 'JustinBieberVEVO', 'provenance': [{'wikipedia_id': '40199886', 'title': 'List of most-subscribed YouTube channels', 'start_paragraph_id': 4, 'start_character': 414, 'end_paragraph_id': 4, 'end_character': 430, 'bleu_score': 1.0, 'section': 'Section::::Most-subscribed channels.\n'}]}]