Which structure causes the hair to "stand on end"?

Rationale:
A. These joints are immovable (e.g., sutures of the skull) and do not allow versatility of movement.
B. Synovial joints are the most common type in the body and allow the greatest range of motion. They are characterized by a joint cavity filled with synovial fluid, articular cartilage, and a joint capsule. Examples include the shoulder, knee, hip, and fingers. These joints permit movements such as flexion, extension, rotation, abduction, and adduction, making them highly versatile.
C. These joints are held together by dense connective tissue and are mostly immovable (e.g., sutures, syndesmoses).
D. These joints are joined by cartilage (e.g., pubic symphysis, intervertebral discs) and allow only limited movement.

Rationale:
A. Melanin production protects against ultraviolet (UV) radiation but does not directly prevent bacterial entry.
B. Sensory perception allows detection of touch, pressure, pain, and temperature, but it does not serve as a barrier against pathogens.
C. The skin acts as a physical and chemical barrier, preventing bacteria, viruses, and other pathogens from entering the body. Its intact epidermis, along with secretions such as sweat and sebum, helps maintain this protective function.
D. Filtration is a function associated with organs like the kidneys, not the skin.