Question: who sang knockin on heaven's door originally
[{'answer': 'Bob Dylan', 'provenance': [{'wikipedia_id': '1308626', 'title': "Knockin' on Heaven's Door", 'start_paragraph_id': 1, 'start_character': 41, 'end_paragraph_id': 1, 'end_character': 50, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]