When looking into the extended entity relationship model $($EERM$),$ which of the following statements is not valid about entity supertypes and subtypes?

a$.$ The entity subtype contains common characteristics.

b$.$ The entity supertype contains common characteristics with subtype.

c$.$ An entity supertype is a generic entity type related to one or more entity subtypes.

d$.$ Entity supertype and subtype are based on a hierarchical structure.