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The modern explanation for the metric expansion of space was proposed by physicist Alan Guth in 1979 , while investigating the problem of why no magnetic monopoles are seen today .
 person: Alan Guth
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: location, person, organization 
Text: {0} 
 Entities: Alan Guth 
Answer:
 person: Alan Guth