Correct Answer: B

Rationale: The primary function of the immune system is to fight infections. It identifies and destroys pathogens like bacteria and viruses, protecting the body from harmful substances. While oxygen transportation is carried out by the respiratory system, energy production is mainly the role of the metabolic system, and body temperature regulation is managed by the thermoregulatory system. These functions are separate from the immune system's primary role of defending the body against infections.

Correct Answer: B

Rationale: The correct answer is B: Insulin. Insulin, produced by the pancreas, is the hormone responsible for lowering blood glucose levels. It achieves this by promoting the uptake of glucose into cells, thereby reducing the concentration of glucose in the bloodstream. Choice A, Glucagon, is actually a hormone produced by the pancreas that works to raise blood glucose levels by stimulating the liver to release stored glucose. Choice C, Cortisol, is a hormone produced by the adrenal glands, not the pancreas, and is involved in stress response and regulation of metabolism. Choice D, Melatonin, is a hormone produced by the pineal gland that regulates sleep-wake cycles and is not directly involved in blood glucose level regulation.

Correct Answer: B

Rationale: The primary function of the immune system is to fight infections. It identifies and destroys pathogens like bacteria and viruses, protecting the body from harmful substances. While oxygen transportation is carried out by the respiratory system, energy production is mainly the role of the metabolic system, and body temperature regulation is managed by the thermoregulatory system. These functions are separate from the immune system's primary role of defending the body against infections.

Correct Answer: A

Rationale: The larynx, also known as the voice box, is a crucial structure in the respiratory system. It is located in the upper part of the trachea and houses the vocal cords. The larynx plays a key role in producing sound and protecting the trachea by closing during swallowing to prevent food from entering the airway. This makes the larynx the correct answer in this context. The trachea (Choice B) is a respiratory structure that connects the larynx to the lungs, allowing air to pass through. The pharynx (Choice C) is a shared pathway for both air and food, located at the back of the throat. The esophagus (Choice D) is part of the digestive system, responsible for transporting food from the mouth to the stomach. Therefore, Choices B, C, and D are not located in the respiratory system.

Correct Answer: A

Rationale: The correct answer is Vitamin C. Vitamin C is essential for the synthesis of collagen, a protein crucial for maintaining the structure and integrity of skin, blood vessels, and bones. It plays a key role in the hydroxylation of proline and lysine residues in collagen synthesis, which is necessary for the stability and strength of collagen fibers. Choice B: Vitamin D is essential for bone health and calcium absorption but is not directly involved in collagen synthesis. Choice C: Vitamin B12 is important for red blood cell formation and neurological function but is not directly related to collagen synthesis. Choice D: Vitamin K is necessary for blood clotting and bone metabolism but does not play a direct role in collagen synthesis.