Correct Answer: C

Rationale: Oligodendrocytes are a type of glial cell that provides structural support and insulation for neurons in the central nervous system by forming myelin sheaths around axons. Schwann cells (option A) perform a similar function but in the peripheral nervous system. Astrocytes (option B) are involved in providing nutrients to neurons, maintaining the chemical environment, and repairing damage. Microglia (option D) are primarily responsible for immune defense in the central nervous system, not for providing structural support and insulation for neurons.

Correct Answer: A

Rationale: Mass is a measure of the amount of matter in an object and remains constant regardless of changes in gravity. Weight, on the other hand, is a measure of the gravitational force acting on an object and varies with changes in gravity. Volume is the amount of space occupied by an object and can change with changes in gravity. Density is the mass of a substance per unit volume and can also change with changes in gravity if the volume changes. Therefore, the correct answer is mass as it remains the same regardless of the gravitational force acting on the object, making it an intrinsic property of the substance.

Correct Answer: A

Rationale: Insulin is a hormone that regulates blood sugar levels by facilitating the uptake of glucose into cells for energy production or storage. The pancreas, located behind the stomach, is the organ responsible for producing insulin. Within the pancreas, specialized cells known as beta cells synthesize and secrete insulin in response to changes in blood glucose levels. The liver, although vital for metabolism, does not produce insulin. Similarly, the stomach and kidneys do not play a role in insulin production. Hence, the correct answer is the pancreas.

Correct Answer: C

Rationale: Sebum is the waxy or oily substance produced by sebaceous glands. It helps lubricate the skin and hair, keeping them moisturized and protected. Keratin is a protein that makes up hair, skin, and nails, providing structure and strength. Melanin is a pigment that gives color to the skin, hair, and eyes, providing protection against UV radiation. Sweat is a watery fluid produced by sweat glands to help regulate body temperature by cooling the body through evaporation.

Correct Answer: C

Rationale: Sweat glands play a vital role in regulating body temperature by releasing a watery fluid that evaporates from the skin's surface. This evaporation process helps cool the body down by dissipating heat, which is essential for maintaining a stable internal temperature. Choice A is incorrect because sweat glands do not produce sebum but rather a watery fluid. Choice B is incorrect because sweating itself does not involve the constriction of blood vessels; instead, it promotes heat loss through evaporation. Choice D is incorrect as sweat glands do not thicken the epidermis for insulation but rather facilitate the cooling process through sweat evaporation.