The number of units selectively representing the entire population is called

Correct Answer: D

Rationale: Sample size is the number of units chosen to represent a population. 'Sample size,' is correct, per statistical design (e.g., Kish), determining study precision (e.g., 100 nurses). 'Sample frame,' is the population list. 'Sampling unit,' is a single entity. 'Sampling fraction,' is the proportion sampled. Sample size's focus on the count of representatives distinguishes D as the accurate answer.

Correct Answer: A

Rationale: Valves in blood vessels prevent backflow, aiding venous return against gravity, but capillaries lack them due to their role in exchange, not directional flow. 'Capillaries,' is correct, as per anatomy texts like Moore's Clinically Oriented Anatomy, which note valves in veins and some venules. 'Veins,' have valves, especially in limbs. 'Venules,' may have valves in larger ones transitioning to veins. 'Vena cavae,' lack valves, but the question's focus on typical presence points to capillaries' exception. Capillaries' thin walls and continuous flow for nutrient exchange exclude valves, making A the accurate answer, distinct from the venous system's structural adaptations.

Correct Answer: B

Rationale: Sperm, the male reproductive cells, are produced in the seminiferous tubules of the testes. 'Seminiferous tubules,' is correct, per reproductive physiology texts like Tortora, where spermatogenesis occurs. 'Epididymis,' stores and matures sperm. 'Vas deferens,' transports sperm. 'Prostate gland,' produces seminal fluid. The tubules' role as the site of sperm cell formation distinguishes B as the accurate answer, foundational to male fertility.

Correct Answer: C

Rationale: Vitamin K is essential for synthesizing clotting factors (e.g., prothrombin) in the liver. 'Vitamin K,' is correct, per physiology texts like Guyton, preventing bleeding disorders. 'Vitamin A,' aids vision. 'Vitamin B,' encompasses metabolic coenzymes, not clotting. 'Vitamin D,' regulates calcium. Vitamin K's specific role in coagulation cascades makes C the accurate answer.

Correct Answer: C

Rationale: The urinary bladder holds about 600 ml at full capacity, though urge begins earlier. '600 ml,' is correct, per physiology texts like Guyton, varying (400-700 ml) by individual. '200 ml,' is minimal. '400 ml,' is functional but not maximum. '800 ml,' exceeds average. The 600 ml norm balances anatomy and function, making C the precise answer.