Question: when did charles dickens write his last book
[{'answer': 'April 1870 to September 1870', 'provenance': [{'wikipedia_id': '5884', 'title': 'Charles Dickens', 'start_paragraph_id': 121, 'start_character': 73, 'end_paragraph_id': 121, 'end_character': 101, 'bleu_score': 1.0, 'section': 'Section::::Notable works.\n'}]}]