(a) Ball and socket joint is a movable joint. It consists of a bone that has a round head, which fits into a cup like depression of another bone. This helps the bone to rotate freely. Examples of such joints are hip and shoulder joints.
(b) Mandible bone which forms the lower jaw is the only skull bone that is movable.
(c) The elbow has a hinge joint. This type of a joint allows the movement in one plane only.
Therefore, our elbow cannot move backwards.