Please help me in filling up the following blank :-

Of the Ganga and the Brahmaputra,which one is the___________? (longer or longest)

The answer is 'longest' because although only two rivers are compared, the word "the" is used only along with superlative form of the word. For example, we say "which one is the longest" and not "which one is the longer". But we can say "the longer river" because the words "the longer" are followed by the noun "river".