Please select each statement that is true in the case of modeling actors in the system.

a$.$ Use cases are always started by an actor.

b$.$ Use cases are only started by actors who are human beings.

c$.$ Use cases are always written from the point of view of the actors.

d$.$ A use case is a specification of sequences of actions, including variant sequences and error sequences, that a system, subsystem or class can perform by interacting with outside actors