Correct Answer: D

Rationale: An atom is the smallest unit of matter that retains the chemical properties of an element. It consists of a nucleus containing protons and neutrons, surrounded by electrons that orbit the nucleus. The number of protons in the nucleus defines the atomic number of the element, and the number of neutrons determines the isotope of the element. While electrons, protons, and neutrons are fundamental particles, an atom as a whole is the smallest part of an element that maintains its unique identity. Electrons are negatively charged particles orbiting the nucleus, protons are positively charged particles in the nucleus, and neutrons are neutral particles in the nucleus. Therefore, the correct answer is 'D: Atom.'

Correct Answer: B

Rationale: The mass number of an element represents the total number of protons and neutrons in an atom's nucleus. It specifically focuses on the number of neutrons in an atom's nucleus, which contributes to the overall mass of the atom. Therefore, the mass number is not solely determined by the number of protons, electrons, or isotopes. Choice A is incorrect because it refers to the atomic number, which is the number of protons in an atom's nucleus. Choice C is incorrect as the number of electrons in the valence shell is related to the atom's reactivity and bonding behavior, not the mass number. Choice D is incorrect because the mass number does not represent the number of isotopes of an element, but rather the sum of protons and neutrons in an atom's nucleus. Thus, option B, the number of neutrons in an atom's nucleus, is the correct choice.

Correct Answer: B

Rationale: Sodium chloride (table salt) is typically a good conductor of electricity when dissolved in water because it dissociates into ions (sodium cations and chloride anions) that can carry an electric current. Sugar (choice A), olive oil (choice C), and oxygen gas (choice D) do not dissociate into ions in water and therefore are not good conductors of electricity. Sugar does not dissociate into ions in water, so it cannot conduct electricity. Olive oil and oxygen gas are nonpolar substances and do not ionize in water, making them poor conductors of electricity.

Correct Answer: A

Rationale: The correct answer is A, Pepsin. Pepsin is primarily responsible for breaking down proteins in the stomach. It is produced in an inactive form called pepsinogen, which becomes activated by the acidic environment in the stomach. Pepsin functions by breaking down proteins into smaller peptides, which are further digested by other enzymes in the small intestine. Lipase is responsible for breaking down fats, amylase for carbohydrates, and maltase for converting maltose into glucose. Therefore, choices B, C, and D are incorrect as they are associated with breaking down fats, carbohydrates, and converting maltose, respectively, not proteins.

Correct Answer: B

Rationale: The Hardy-Weinberg equilibrium describes a theoretical population in which allele frequencies remain constant from generation to generation, indicating that the population is not evolving. This equilibrium occurs under specific conditions: no mutation, no gene flow, random mating, a large population size, and no natural selection. In this scenario, all genotypes are in proportion to the allele frequencies, and genetic diversity is maintained. Options A, C, and D do not accurately describe a population in Hardy-Weinberg equilibrium. Option A suggests rapid evolution due to strong directional selection, which would disrupt the equilibrium. Option C mentions a founder effect, which can reduce genetic diversity but is not a characteristic of a population in Hardy-Weinberg equilibrium. Option D describes a population dominated by a single homozygous genotype, which also does not align with the genetic diversity seen in a population at Hardy-Weinberg equilibrium.