Detailed Instructions: This task is about creating an unanswerable question based on a given passage. Construct a question that looks relevant to the given context but is unanswerable. Following are a few suggestions about how to create unanswerable questions:
(i) create questions which require satisfying a constraint that is not mentioned in the passage
(ii) create questions which require information beyond what is provided in the passage in order to answer
(iii) replace an existing entity, number, date mentioned in the passage with other entity, number, date and use it in the question
(iv) create a question which is answerable from the passage and then replace one or two words by their antonyms or insert/remove negation words to make it unanswerable.
Problem:Passage: As of 1878, there were only three free Slavic states in the world: the Russian Empire, Serbia and Montenegro. Bulgaria was also free but was de jure vassal to the Ottoman Empire until official independence was declared in 1908. In the entire Austro-Hungarian Empire of approximately 50 million people, about 23 million were Slavs. The Slavic peoples who were, for the most part, denied a voice in the affairs of the Austro-Hungarian Empire, were calling for national self-determination. During World War I, representatives of the Czechs, Slovaks, Poles, Serbs, Croats, and Slovenes set up organizations in the Allied countries to gain sympathy and recognition. In 1918, after World War I ended, the Slavs established such independent states as Czechoslovakia, the Second Polish Republic, and the State of Slovenes, Croats and Serbs.
Solution:
What was Serbia de jure vassal to in 1878?