The primary function of flat bones, such as the scapula and cranial bones, is to:

Correct Answer: C

Rationale: Flat bones, like the scapula (shoulder blade) and cranial bones, are thin and broad, providing a protective barrier for internal organs. The cranial bones protect the brain, while the scapula contributes to the shoulder structure, indirectly aiding in upper body protection. Although flat bones like those in the skull and sternum also produce blood cells in their marrow, their primary function is protection, making 'Protect internal organs' the correct answer.

Correct Answer: C

Rationale: Acetylcholine (ACh) is the neurotransmitter released by motor neurons into the synaptic cleft at the neuromuscular junction. It binds to receptors on the muscle fiber, initiating an action potential that leads to contraction. Serotonin, GABA, and dopamine have other neural roles, making 'Acetylcholine (ACh)' the correct answer.

Correct Answer: B

Rationale: In a relaxed muscle, tropomyosin covers the myosin-binding sites on actin filaments, preventing contraction. When calcium binds to troponin, it moves tropomyosin, exposing the sites for myosin attachment. Troponin regulates, myosin contracts, and collagen is structural, making 'Tropomyosin' the correct answer.

Correct Answer: B

Rationale: The metacarpal bones, found in the hand, are part of the appendicular skeleton, which includes the limbs and girdles. They are long bones but belong to the appendicular division, not axial (skull, spine, ribs). 'Long bones' is a type, not a division, making 'Appendicular skeleton' correct.

Correct Answer: C

Rationale: The humerus, an upper arm bone in the appendicular skeleton, facilitates arm movement by articulating with the scapula and forearm bones (radius, ulna). It doesn’t protect the heart (rib cage’s role) or support lower limbs, making 'Facilitating arm movement' the correct answer.