Question: when was mercaptan first added to natural gas
[{'answer': '1937', 'provenance': [{'wikipedia_id': '15424786', 'title': 'Odorizer', 'start_paragraph_id': 21, 'start_character': 329, 'end_paragraph_id': 21, 'end_character': 333, 'bleu_score': 1.0, 'section': 'Section::::Liquid injection type odorizers.\n'}]}]