Given the task definition and input, reply with output. 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.

Passage: In 1954, major Serbian and Croatian writers, linguists and literary critics, backed by Matica srpska and Matica hrvatska signed the Novi Sad Agreement, which in its first conclusion stated: "Serbs, Croats and Montenegrins share a single language with two equal variants that have developed around Zagreb (western) and Belgrade (eastern)". The agreement insisted on the equal status of Cyrillic and Latin scripts, and of Ekavian and Ijekavian pronunciations. It also specified that Serbo-Croatian should be the name of the language in official contexts, while in unofficial use the traditional Serbian and Croatian were to be retained. Matica hrvatska and Matica srpska were to work together on a dictionary, and a committee of Serbian and Croatian linguists was asked to prepare a pravopis. During the sixties both books were published simultaneously in Ijekavian Latin in Zagreb and Ekavian Cyrillic in Novi Sad. Yet Croatian linguists claim that it was an act of unitarianism. The evidence supporting this claim is patchy: Croatian linguist Stjepan Babic complained that the television transmission from Belgrade always used the Latin alphabet which was true, but was not proof of unequal rights, but of frequency of use and prestige. Babic further complained that the Novi Sad Dictionary (1967) listed side by side words from both the Croatian and Serbian variants wherever they differed, which one can view as proof of careful respect for both variants, and not of unitarism. Moreover, Croatian linguists criticized those parts of the Dictionary for being unitaristic that were written by Croatian linguists. And finally, Croatian linguists ignored the fact that the material for the Pravopisni rjecnik came from the Croatian Philological Society. Regardless of these facts, Croatian intellectuals brought the Declaration on the Status and Name of the Croatian Literary Language in 1967. On occasion of the publication's 45th anniversary, the Croatian weekly journal Forum published the Declaration again in 2012, accompanied by a critical analysis.
What did Serbian writers and linguists sign in 1967?