In the extended entity$-$relationship model $($EERM$),$ which of the following statements best describes the completeness constraint?

a$.$ The completeness constraint specifies whether each entity supertype occurrence must also be a member of at least one subtype.

b$.$ Partial completeness constraint means that every supertype occurrence is a member of a subtype.

c$.$ Total completeness constraint means that every subtype occurrence must be a member of at least one supertype.

d$.$ The completeness constraint specifies whether each entity subtype occurrence must also be a member of at least one supertype.