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For presidents Ronald Reagan , Bill Clinton , George W. Bush and Barack Obama , for example , their respective paths to nomination became uneventful and the races become merely " pro forma " ; all four then went on to win a second presidential term .
 Ronald Reagan, Bill Clinton, George W. Bush, Barack Obama
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Ronald Reagan, Bill Clinton, George W. Bush, Barack Obama