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In addition , we found that an oxygen extraction reaction by a CO molecule actually occurs for oxygen - rich clusters such as Ni6O7 ( + ) and Ni8O9 ( + ) .
 Chemical: oxygen; Chemical: CO; Chemical: oxygen; Chemical: Ni6O7 ( + ); Chemical: Ni8O9 ( + )
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: oxygen; Chemical: CO; Chemical: oxygen; Chemical: Ni6O7 ( + ); Chemical: Ni8O9 ( + )