## 2 Background
### Linked Data  
Data on the Semantic Web [29] are represented following the conventions of Resource Description Framework (RDF) [32] standard, using triples format. Triples are each constituted by a subject, a predicate and an object. To express these, several standardised languages have been developed, such as RDF XML [33], Turtle [34], JSON-LD [35], etc. Furthermore, RDF data can be queried using the semantics and syntax of standardised and well known RDF query languages, such as SPARQL [36]. Additionally, a set of vocabularies known as ontologies enable to describe, represent and relate terms pursuant to their domain. Typically, each ontology represents an area of concern by providing controlled vocabularies of concepts with machine-readable semantics. There is a large number of widely used ontologies to describe different fields of knowledge. For example, the International Committee for Documentation Conceptual Reference Model (CIDOC CRM) ontology has been internationally adopted to enable data interchange of cultural heritage and it has become an ISO standard since 2006 [37]. To further highlight the extent of the aforementioned ontology's relevance, the renowned (In 2018 Prado Museum was ranked amongst the top 15 most visited European museums [[https://www.statista.com/statistics/747942/attendance-at-leading-museums-in-europe/](https://www.statista.com/statistics/747942/attendance-at-leading-museums-in-europe/)]([https://www.statista.com/statistics/747942/attendance-at-leading-museums-in-europe/](https://www.statista.com/statistics/747942/attendance-at-leading-museums-in-europe/))) Prado Museum in Spain underwent a digital transformation in 2016 [38] resulting in the publication of their online collection as an RDF graph. The model was described using International Committee for Documentation (CIDOC) Conceptual Reference Model (CRM) vocabularies. Other examples of prominent museums having their holdings published as Linked Data based on CIDOC CRM ontology include: The British Museum [39] and The National Gallery [40]. It must be taken into account that, in the cultural heritage domain, there are also other ontologies and schemas used, such as Descriptive Ontology for Linguistic and Cognitive Engineering (DOLCE) [41] and Dublin Core (DC) [42]. DOLCE is, like CIDOC CRM, a global ontology consisting of general terms that can be applied across different domains. This ontology was not specifically conceived to describe and represent cultural heritage, however, since it aims to provide a formal description of a particular conceptualization of the world [43], DOLCE is also used in this area. In addition, DC schema provides a small list of vocabularies that can be used to describe metadata about a given item. Given that an ontology usually refers to a semantically complex set of vocabularies with rich relationships between them, DC is considered a metadata element set or schema because it is a rather simple ontology. This simplicity is sometimes, in fact, a desirable trait when describing cultural heritage because some properties of exhibits require expert knowledge [44] in order to be correctly described using CIDOC CRM or DOLCE vocabularies. DC can offer a more straightforward, yet less accurate alternative. Furthermore, there are also hierarchical collections of words known as thesauri. These aim to help index and extract terms classified into a particular topic or area of knowledge. For instance, UNESCO offers a thesaurus that intends to contribute towards helping to analyse and retrieve documents and publications in several fields, including education and culture [45]. This thesaurus is constructed based on the SKOS taxonomy [46], which, unlike ontologies, it focuses on organising knowledge instead of representing it. Provided the machine-readable semantics brought about by Linked Data, one of the many approaches towards harnessing these data consists in the automation of knowledge extractionprocedures. A large number of fields enjoy the benefits of such automation, as is the case of question generation schemes.