Question: who played the role of joker in batman dark knight
[{'answer': 'Heath Ledger', 'provenance': [{'wikipedia_id': '21145395', 'title': 'Joker (The Dark Knight)', 'start_paragraph_id': 1, 'start_character': 213, 'end_paragraph_id': 1, 'end_character': 225, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]