Question: the element 90sr undergoes beta decay to form which isotope
[{'answer': 'yttrium - 90'}, {'provenance': [{'wikipedia_id': '5700197', 'title': 'Strontium-90', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 324, 'bleu_score': 0.771853481978062, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13686064392904205087}, 'section': 'Section::::Abstract.'}]}]