Concept:The Rio de la Plata (River Plate) forms the border between Argentina and Uruguay, with the capitals Buenos Aires and Montevideo on opposite banks.
Explanation:The River Plate is located on the east coast of South America, separating Argentina and Uruguay.
Buenos Aires, the capital of Argentina, lies on its western shore.
Montevideo, the capital of Uruguay, lies on its northern shore.
Hence, both cities are situated across the banks of the River Plate.
Answer:River Plate