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Members of the Legislative Yuan questioned Ma 's decision , as Chen had previously been a U.S. citizen and had also obtained a U.S. green card again for a short period while serving on the Supreme Court ( see personal life , below ) .
 Ma
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Ma