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The concept of mandatory euthanasia for humans after a " fixed period " ( often 60 years ) became a recurring theme in 20th century imaginative literature - for example , Isaac Asimov 's 1950 novel " Pebble in the Sky " .
 person: Isaac Asimov
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 person: Isaac Asimov