I want to know the answer to the following question:
Which tributary is located on the southern border?
All the background I'm provided with is that "The most important river in Thuringia is the Saale (a tributary of the Elbe) with its tributaries Unstrut, Ilm and Weiße Elster, draining the most parts of Thuringia and the Werra (the headwater of the Weser), draining the south-west and west of the Land.". Is that enough to answer the question?
Answer: no