Correct Answer: B

Rationale: Rationale: A) Conjugation is a process in which genetic material is transferred between two bacterial cells through a conjugation tube. It does not involve the division of a single-celled organism into two identical daughter cells. B) Binary fission is the process by which a single-celled organism, such as a bacterium, divides into two identical daughter cells. This is a common method of reproduction in prokaryotic organisms. C) Endocytosis is a cellular process in which cells engulf external materials by wrapping them in a vesicle. It is not a method of cell division. D) Mitosis is a type of cell division that occurs in eukaryotic cells, leading to the production of two daughter cells with identical genetic material. However, the question specifically refers to a single-celled organism, which typically undergoes binary fission for reproduction.

Correct Answer: C

Rationale: In nuclear physics, the term 'barn' is a unit used to quantify nuclear cross-section. Nuclear cross-section is a measure of the probability of a nuclear reaction occurring when an atomic nucleus interacts with a particle or another nucleus. The barn is a unit of area equal to 10^-28 square meters; it is commonly used to describe the cross-sectional area of atomic nuclei for nuclear reactions. Choice A, 'Energy,' is incorrect because a barn is not a unit for measuring energy; it is a unit of area. Choice B, 'Radioactivity,' is incorrect as radioactivity is typically measured in units like becquerels. Choice D, 'Half-life,' is also incorrect as half-life is a measure of the time it takes for half of a substance to decay, not related to the concept of a barn as a unit of nuclear cross-section.

Correct Answer: A

Rationale: Rationale: A) To protect the body from harm: The skin acts as a physical barrier that protects the body from external threats such as pathogens, UV radiation, and physical injuries. It helps prevent infections and dehydration, making it a crucial function of the skin. B) To regulate body temperature: While the skin does play a role in regulating body temperature through processes like sweating and vasodilation/vasoconstriction, its primary function is protection. C) To produce hormones: Hormone production is primarily carried out by endocrine glands such as the pituitary gland, thyroid gland, and adrenal glands, not the skin. D) To sense touch, temperature, and pain: The skin contains sensory receptors that allow us to perceive touch, temperature, and pain, but this function is secondary to its main role of protecting the body from harm.

Correct Answer: B

Rationale: Valence electrons are the electrons in the outermost energy level of an atom. These electrons are involved in forming bonds with other atoms, which is crucial for chemical bonding. By participating in bonding, valence electrons determine an atom's ability to form compounds and engage in chemical reactions. Therefore, the primary function of valence electrons is to facilitate the formation of bonds between atoms, making option B the correct answer. Choices A, C, and D are incorrect because valence electrons primarily influence chemical bonding by participating in the formation of bonds between atoms, rather than holding the nucleus together, determining physical properties, or having no role in chemical reactions.

Correct Answer: A

Rationale: The correct answer is A: Myelin sheath. The myelin sheath is a fatty layer that surrounds and insulates some nerve fibers, facilitating the rapid transmission of signals along the nerve fibers. Choices B, C, and D (Dura mater, Pia mater, and Arachnoid mater) are layers of the meninges, protective membranes that surround the brain and spinal cord. The primary function of the meninges is to protect and support the central nervous system, not to insulate nerve fibers for signal transmission.