Which of the following statements accurately describes the differences or similarities between RB$-$RBAC and ABAC? Select all that apply

a$.$ RB$-$RBAC rules can be formatted using an If$-$Then$-$Else structure.

b$.$ The RB$-$RBAC scheme can dynamically assign roles to subjects based on a set of rules.

c$.$ Both RBAC and RB$-$RBAC rules can be modified by users under very specific circumstances.

d$.$ ABAC uses flexible policies that can combine attributes.