Final answer:
The scapula is a flat bone that needs to be mobile for arm movement, and it is supported by the clavicle as part of the pectoral girdle.
Step-by-step explanation:
Since the scapula is a flat bone, it must be mobile in order for the arm to be able to move. The scapula, or shoulder blade, is part of the pectoral girdle and is crucial in anchoring the upper limb to the body. It lies on the posterior side of the shoulder and articulates with the humerus to form the shoulder joint. The clavicle, or collarbone, supports the scapula and is attached to the sternum, providing a sturdy yet mobile base that allows for a wide range of motion. The scapula's mobility is essential for the many movements of the shoulder and arm, facilitated by its muscular attachments and the lack of direct articulation with the ribs.