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In 1978 , Zeldovich noted the monopole problem , which was an unambiguous quantitative version of the horizon problem , this time in a subfield of particle physics , which led to several speculative attempts to resolve it .
 Zeldovich
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Zeldovich