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The 8 - week - old rats exhibited higher levels of general activity and exploration during the training and performed best with respect to spatial learning and memory , whereas no difference was found between the 12 - week - old and 24 - week - old rats .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 None