Question: give the name of a substance that is used as a fuel in the bunsen burner
[{'answer': 'natural gas', 'provenance': [{'wikipedia_id': '4924', 'title': 'Bunsen burner', 'start_paragraph_id': 2, 'start_character': 15, 'end_paragraph_id': 2, 'end_character': 26, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'propane', 'provenance': [{'wikipedia_id': '4924', 'title': 'Bunsen burner', 'start_paragraph_id': 2, 'start_character': 91, 'end_paragraph_id': 2, 'end_character': 98, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'butane ,', 'provenance': [{'wikipedia_id': '4924', 'title': 'Bunsen burner', 'start_paragraph_id': 8, 'start_character': 173, 'end_paragraph_id': 8, 'end_character': 183, 'bleu_score': 1.0, 'section': 'Section::::Operation.\n'}]}, {'provenance': [{'wikipedia_id': '4924', 'title': 'Bunsen burner', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 128, 'bleu_score': 0.9181891462193901, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 1236049677475606023}, 'section': 'Section::::Abstract.'}]}]