What river flows through Germany, Austria, Slovakia, and Hungary?

Which UNESCO World Heritage site is located in the Russian Far East and features a unique volcanic landscape? What is the name of the UNESCO World Heritage site located in the Namib Desert, known for its unique sand dunes? What river flows through China, Laos, Thailand, Cambodia, and Vietnam? What river flows through Russia, Kazakhstan, and China? 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 runs along the border between Romania and Bulgaria?