Question: who is the author of book entitled principles of economics
[{'answer': 'Alfred Marshall', 'provenance': [{'wikipedia_id': '12886649', 'title': 'Principles of Economics (Marshall)', 'start_paragraph_id': 1, 'start_character': 80, 'end_paragraph_id': 1, 'end_character': 95, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '12886649', 'title': 'Principles of Economics (Marshall)', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 223, 'bleu_score': 0.843133901932949, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11760792913299186385}, 'section': 'Section::::Abstract.'}]}]