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Daniel Friedan , Emil Martinec and Stephen Shenker further developed the covariant quantization of the superstring using conformal field theory techniques .
 Daniel Friedan, Emil Martinec, Stephen Shenker
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Daniel Friedan, Emil Martinec, Stephen Shenker