Slow oxidative fibers are suited for endurance-type activities.

Choice A reason: The buccinator muscle compresses the cheek, aiding in actions like blowing and chewing. While it contributes to facial expression, it is not the primary muscle responsible for smiling.

Choice B reason: The zygomaticus muscle is the correct answer. It originates from the zygomatic bone and inserts into the corner of the mouth. Its contraction pulls the mouth upward and outward, producing a smile. It is the principal muscle involved in smiling.

Choice C reason: The mentalis muscle elevates and wrinkles the skin of the chin and protrudes the lower lip. It is involved in expressions of doubt or displeasure, not smiling.

Choice D reason: The orbicularis oris encircles the mouth and controls movements of the lips, such as puckering or closing. It is essential for speech and kissing but not for smiling.

Choice E reason: The masseter is a major muscle of mastication (chewing). It elevates the mandible to close the jaw and has no role in facial expressions like smiling.

Choice A reason: Hinge joints allow movement in one plane, such as flexion and extension. Examples include the elbow and knee, but the joint between the radius and ulna does not function this way.

Choice B reason: This is correct. The proximal radioulnar joint is a pivot joint, allowing rotational movement such as pronation and supination of the forearm. The radius rotates around the ulna, enabling the palm to turn up or down.

Choice C reason: Plane or gliding joints allow sliding movements between flat surfaces, such as those between carpal bones. This does not apply to the radius and ulna.

Choice D reason: Saddle joints allow movement in two planes and are found in the thumb (carpometacarpal joint). The radius and ulna do not form a saddle joint.

Choice E reason: Ball-and-socket joints allow movement in multiple directions and are found in the shoulder and hip. The radius and ulna do not form this type of joint.