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Much of the southern border is formed by tributaries of the Congo River , with the Mbomou River in the east merging with the Uele River to form the Ubangi River .
 location: Congo River; location: Mbomou River; location: Uele River; location: Ubangi River
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: location, person, organization 
Text: {0} 
 Entities: Congo River, Mbomou River, Uele River, Ubangi River 
Answer:
 location: Congo River; location: Mbomou River; location: Uele River; location: Ubangi River