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Manmohan Singh succeeded Vajpayee as Prime Minister as the head of the United Progressive Alliance .
 Manmohan Singh, Vajpayee, United Progressive Alliance
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Manmohan Singh, Vajpayee, United Progressive Alliance