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Many interesting and mysterious features have been discovered in the Hellas quadrangle , including the giant river valleys Dao Vallis , Niger Vallis , Harmakhis , and Reull Vallis — all of which may have contributed water to a lake in the Hellas basin in the distant past .
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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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