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Participants inhaled air 7 . 5 % CO ( 2 ) for 20 minutes ( single - blind presentation ) , and a non - blinded single vital capacity of 35 % CO ( 2 ) .
 CO ( 2 ), CO ( 2 )
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 CO ( 2 ), CO ( 2 )