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The radioactive-Earth theme was begun in " Pebble in the Sky " , which is set thousands of years earlier .
 Pebble in the Sky
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:
 Pebble in the Sky