Which of the following statements is incorrect?

a$.$ A key$-$value $($KV$)$ database is a NoSQL database model that stores data as a collection of key$-$value pairs in which the value component is unintelligible to the DBMS$.$

b$.$ A document database is a NoSQL database model that stores data in key$-$value pairs in which the value component is composed of a tag$-$encoded document.

c$.$ A column family database is a NoSQL database model that organizes data into key$-$value pairs, in which the value component is composed of a set of rows that vary by column.

d$.$ A graph database is a NoSQL database model based on graph theory that stores data on relationship$-$rich data as a collection of nodes and edges.