Question: who sang old man river in movie showboat
[{'answer': 'Paul Robeson', 'provenance': [{'wikipedia_id': '1002449', 'title': "Ol' Man River", 'start_paragraph_id': 3, 'start_character': 251, 'end_paragraph_id': 3, 'end_character': 263, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'William Warfield', 'provenance': [{'wikipedia_id': '1002449', 'title': "Ol' Man River", 'start_paragraph_id': 11, 'start_character': 127, 'end_paragraph_id': 11, 'end_character': 143, 'bleu_score': 1.0, 'section': "Section::::Paul Robeson's alterations to the song lyrics.\n"}]}]