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From 2006 to 2008 , he was chief energy economist at Lehman Brothers , where he argued the oil price rises of 2007 and 2008 were an unsustainable bubble .
 Lehman Brothers
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Lehman Brothers