Question: when did the hoover dam started being built
[{'answer': '1931', 'provenance': [{'wikipedia_id': '14308', 'title': 'Hoover Dam', 'start_paragraph_id': 1, 'start_character': 173, 'end_paragraph_id': 1, 'end_character': 177, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]