Which river forms part of the border between Namibia and Angola?

Which river runs along the border between Zambia and Zimbabwe? Which river forms part of the border between Namibia and South Africa? Which river runs along the border between Romania and Bulgaria? Which river forms part of the border between Argentina and Uruguay? Which river flows through Brazil, Peru, and Colombia? Which river forms part of the border between Thailand and Laos? Which river flows through Argentina, Bolivia, and Paraguay? Which river forms part of the border between Croatia and Serbia?