The division of the skeletal system responsible for providing support and protection to the body's major organs, such as the heart and lungs, is the:

Correct Answer: A

Rationale: The axial skeleton, including the skull, vertebral column, and rib cage, is designed to support and protect major organs. The rib cage shields the heart and lungs, the skull protects the brain, and the vertebral column encases the spinal cord. The appendicular skeleton focuses on movement, while irregular and long bones are types, not divisions, making 'Axial skeleton' the correct answer.

Correct Answer: A

Rationale: In the sliding filament theory, calcium ions trigger myosin binding to actin. Released from the sarcoplasmic reticulum, calcium binds to troponin, shifting tropomyosin to expose actin’s binding sites, allowing myosin heads to attach. Sodium and potassium regulate nerve signals, not this process, making 'Calcium ions' the correct answer.

Correct Answer: B

Rationale: Short bones, like the carpals (wrist) and tarsals (ankle), are roughly equal in length and width, providing stability and support to joints while allowing some movement. Long bones are in limbs, irregular bones have complex shapes, and flat bones are protective, making 'Short bone' the correct answer.

Correct Answer: C

Rationale: The skull, part of the axial skeleton, forms a bony encasement around the brain, protecting it from injury. The vertebral column protects the spinal cord, the rib cage shields thoracic organs, and the pelvic girdle is appendicular, making 'Skull' the correct answer.

Correct Answer: B

Rationale: When muscle fibers contract, sarcomeres (the units between Z-lines) shorten as actin and myosin filaments slide past each other, per the sliding filament theory. This reduces sarcomere length, contracting the muscle, making 'Sarcomeres shorten' the correct answer.