Correct Answer: D

Rationale: The Gram stain is a differential staining technique that categorizes bacteria into two groups based on their cell wall composition: Gram-positive and Gram-negative. Gram-positive bacteria have a thick layer of peptidoglycan in their cell walls, which retains the crystal violet stain used in the Gram staining procedure. Consequently, Gram-positive bacteria appear purple under the microscope after staining. On the other hand, Gram-negative bacteria possess a thinner peptidoglycan layer and an outer membrane that can be penetrated by the counterstain safranin, leading them to appear pink or red. Blue is not typically used to describe the color of bacteria in a Gram stain, making it an incorrect choice.

Correct Answer: B

Rationale: The external body covering, which is the skin, is composed of epithelial tissue. Epithelial tissue serves as a protective barrier against pathogens, chemicals, and physical damage. It also helps regulate body temperature through processes like sweating. Choice A (Muscle tissue) is incorrect as muscles are responsible for movement and not the external body covering. Choice C (Bone marrow) is incorrect as it is a component of the skeletal system and not the external body covering. Choice D (Cartilage) is incorrect as cartilage is a type of connective tissue found in joints and the structure of some body parts, not the external body covering.

Correct Answer: B

Rationale: Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Since the atomic mass of an element is the weighted average of the masses of its isotopes, the presence of isotopes causes the atomic mass of an element to vary slightly. This variation occurs because different isotopes have different masses due to their varying numbers of neutrons. The atomic mass is affected by the abundance of each isotope, leading to a slight fluctuation in the overall atomic mass of the element. Choice A is incorrect because isotopes do influence the atomic mass. Choice C is incorrect because isotopes have different masses, affecting the overall atomic mass. Choice D is incorrect as isotopes typically do not cause a significant variation in atomic mass, but rather a slight fluctuation.

Correct Answer: B

Rationale: The Golgi apparatus plays a crucial role in the cell by packaging and modifying proteins and lipids synthesized by the endoplasmic reticulum. These proteins and lipids are then transported to their final destinations within the cell or outside of it. The Golgi apparatus does not break down and recycle cellular waste, generate energy through cellular respiration, or translate DNA into proteins. Its primary function is related to processing and sorting molecules for cellular use.

Correct Answer: C

Rationale: The involuntary reflex arc that withdraws your hand from a hot object involves both the central nervous system (CNS) and the peripheral nervous system (PNS). When your hand touches a hot object, sensory neurons in the PNS detect the heat and send signals to the spinal cord in the CNS. The spinal cord processes this information and sends a signal back through motor neurons in the PNS to move your hand away from the hot object. This coordinated response requires the collaboration of both the CNS and PNS to protect the body from harm. Choice A is incorrect because the reflex arc involves more than just the central nervous system. Choice B is incorrect because the reflex arc involves more than just the peripheral nervous system. Choice D is incorrect because the reflex arc also involves motor neurons, not just sensory neurons.