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There are many rivers in Russia, the longest ones are the Volga in Europe and (the?) Yenisei and
(the?) Ob in Asia.
Baikal water once was (the?) purest in the world.
I see now, sorry- I would repeat the article with the name of each in the list. In the second sentence, there's no need for an article because it's functioning as an adjective.
the Volga in Europe and (the?) Yenisei 
Linear Mode
