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Much of the southern border is formed by tributaries of the Congo River ; the Mbomou River in the east merges with the Uele River to form the Ubangi River , which also comprises portions of the southern border .
 Congo River, Mbomou River, Uele River, Ubangi River
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Congo River, Mbomou River, Uele River, Ubangi River