Which river runs along the border between Romania and Bulgaria?

What river flows through Germany, Austria, Slovakia, and Hungary? Which river forms part of the border between France and Spain? Which river runs along the border between Zambia and Zimbabwe? Which river forms part of the border between Namibia and South Africa? Which river forms part of the border between Argentina and Uruguay? Which river forms part of the border between Namibia and Angola? Which river flows through Brazil, Peru, and Colombia? Which river forms part of the border between Thailand and Laos?