The Sternocleidomastoid (SCM)muscle Is Considered an Accessory Muscle of Respiration

The sternocleidomastoid (SCM) muscle is considered an accessory muscle of respiration.When the head is fixed,the SCM muscle creates an increase in the anteroposterior diameter of the thorax.However,this is not the primary function of this muscle.What does the SCM do when not serving as an accessory respiratory muscle?

A) When one contracts, the head flexes.
B) When one contracts, the head side bends toward the shoulder of the same side and the face rotates opposite.
C) When one contracts, the head rotates and side bends toward the shoulder of the same side.
D) When both contract, the head extends.