Question: how many isotopes of oxygen occur naturally on earth
[{'answer': 'three stable isotopes', 'provenance': [{'wikipedia_id': '2527111', 'title': 'Isotopes of oxygen', 'start_paragraph_id': 4, 'start_character': 42, 'end_paragraph_id': 4, 'end_character': 63, 'bleu_score': 1.0, 'section': 'Section::::Stable isotopes.\n'}]}, {'provenance': [{'wikipedia_id': '2527111', 'title': 'Isotopes of oxygen', 'start_paragraph_id': 4, 'start_character': 0, 'end_paragraph_id': 4, 'end_character': 266, 'bleu_score': 0.696184695134176, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4715323289169906885}, 'section': 'Section::::Stable isotopes.\n'}]}]