Which of the following statements is not correct?

a$.$ An RDF data model consists of statements which are in

subject$$predicate$$object format.

b$.$ RDF allows use of database$-$specific primary keys to identify

resources.

c$.$ An RDF data model can be visualized as a directed, labeled

graph.

d$.$ RDF Schema enriches RDF by extending its vocabulary with

classes and subclasses, properties and subproperties, and typing of

properties