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In 1933 , while studying dentistry at Creighton University , Cassel was provided with the opportunity to sing for renowned baritone Lawrence Tibbett after attending one of Tibbet 's recitals in Omaha , Nebraska .
 Creighton University, Cassel, Lawrence Tibbett, Omaha , Nebraska
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Creighton University, Cassel, Lawrence Tibbett, Omaha , Nebraska