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He decided that it would be a two-volume story arc , as had proved successful with his earlier arcs , " The Secret of the Unicorn " ( 1943 ) and " Red Rackham 's Treasure " ( 1944 ) , and " The Seven Crystal Balls " ( 1948 ) and " Prisoners of the Sun " .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 None