Correct Answer: A

Rationale: The pleurae play a crucial role in compartmentalizing and lubricating the lungs. The pleura is a double-layered membrane that surrounds the lungs and lines the chest cavity. This double-layered structure reduces friction between the lungs and the chest wall during breathing movements, allowing smooth expansion and contraction of the lungs. Options B, C, and D are incorrect. Option B, 'To facilitate the flow of air,' is incorrect because while the pleurae are involved in the respiratory process, their primary function is not to directly facilitate the flow of air. Option C, 'To produce mucus,' is incorrect as the production of mucus is not a primary function of the pleurae. Option D, 'To serve as the passageway for air,' is incorrect as the passageway for air refers to the air passages like the trachea and bronchi, not the pleurae. Therefore, option A is correct as it accurately describes the function of the pleurae.

Correct Answer: D

Rationale: The skin is the largest organ in the human body. It covers the entire body, serving as a protective barrier against pathogens, UV radiation, and physical damage. Additionally, the skin plays a crucial role in regulating body temperature and synthesizing vitamin D when exposed to sunlight. Choices A, B, and C are incorrect. The liver is the largest internal organ, the brain is the control center of the body, and the heart is a vital organ responsible for pumping blood throughout the body. However, none of these organs are the largest in terms of surface area or overall size.

Correct Answer: B

Rationale: The correct answer is B. Alleles are different forms of a gene that can exist at a specific locus. They represent variations of a gene and are inheritable. Choice A is incorrect as it does not fully capture the concept of an allele, which is a specific form of a gene rather than a substitution. Choice C is incorrect as alleles are not a grouping of various genes but rather different forms of a single gene at a specific locus. Choice D is also incorrect as it does not accurately describe alleles; alleles are variations of genes, not base pairs.

Correct Answer: C

Rationale: The correct answer is C: GABA (gamma-aminobutyric acid). GABA acts as an inhibitory neurotransmitter that reduces neuronal activity, thus preventing overexcitation. Acetylcholine (choice A) is involved in muscle control and cognitive function, but it is not primarily responsible for slowing down neuronal activity. Dopamine (choice B) plays a role in reward-motivated behavior and motor control, rather than inhibiting neuronal firing. Serotonin (choice D) is involved in mood regulation, sleep, and appetite but does not primarily slow down neuronal activity to prevent overexcitation.

Correct Answer: A

Rationale: The endoplasmic reticulum is the correct answer. It is the cytoplasmic organelle that contains fluid-filled tubules responsible for carrying and transporting substances within the cell. The endoplasmic reticulum plays a crucial role in protein synthesis, lipid metabolism, and detoxification processes. The Golgi apparatus, although important in modifying, sorting, and packaging proteins for secretion or for use within the cell, does not primarily contain fluid-filled tubules for carrying and transporting substances. Mitochondria are responsible for generating ATP through cellular respiration, not for carrying and transporting substances. The nucleus contains the cell's genetic material and is involved in regulating cellular activities but does not contain fluid-filled tubules for transportation.