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In the 1990 s , Joseph Polchinski discovered that the theory requires higher-dimensional objects , called D-branes and identified these with the black-hole solutions of supergravity .
 person: Joseph Polchinski
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: Joseph Polchinski 
Answer:
 person: Joseph Polchinski