The correct option is A
A river is a natural stream of water that flows with continuity. It has a certain flow, is rarely constant throughout the year, and flows into the sea, into a lake or into another river, in which case it is called a tributary. The final part of a river is its mouth. Sometimes they end up in desert areas where their water is lost by infiltration and evaporation due to the intense temperatures.
Rivers are the most practical way to establish borders, due to their natural character, their stability over time, difficulty of franqueability and the division they geographically make, with regions on one side or the other of their course. Although the rivers are used as border lines, they do not culturally separate the peoples, since through different elements, they maintain constant communication that allows a fluid interaction.