Joint movement occurs around an axis that is always to its plane.

perpendicular

Explanation: Joint can be found in areas of the body where two or more bones are connected together, practical examples in the human body can be traced to our elbow, wrist, hip, knee. At Each of these points mentioned, it could be observed that it serves as a meeting point for two or more bones which are linked and formed around an axis. This joints enables us to conveniently move, twist our rigid bones in different directions perpendicular to the plane (that is in a direction opposite or away from the direction of the plane).

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answer; ///(b) it is composed of four protein chains and four heme groups;