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While the most common type of extension is the C extension , in rare cases , owners of five-string basses , in which the lowest string is normally a low B , may use either a two semitone extension , providing a low A , or the very rare low G extension .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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