Correct Answer: A

Rationale: The correct answer is A because the cloudy surface indicates that the bacteria are growing where oxygen is present (aerobes), causing turbidity. The clear bottom suggests that the bacteria are not growing in the absence of oxygen. Choices B, C, and D are incorrect because the observed phenomena do not necessarily indicate the need for an anaerobic chamber, enzyme deficiencies, or the microorganism being non-aerotolerant.

Correct Answer: C

Rationale: The correct answer is C: after repeated exposure for days. Subacute toxic effects typically manifest after a period of time that is longer than immediate (A) or acute (B) effects but shorter than chronic effects (D). This duration of days aligns with the definition of subacute, which refers to effects that occur gradually over a relatively short time frame. It is important to differentiate between the timing of toxic effects to accurately assess the potential harm caused by a substance. Choices A, B, and D are incorrect because they do not align with the characteristic time frame associated with subacute toxic effects.

Correct Answer: C

Rationale: The correct answer is C: nucleotides. Nucleotides are the structural units of nucleic acids, consisting of a nitrogenous base, a sugar molecule, and a phosphate group. The nitrogenous base can be adenine, thymine, cytosine, guanine, or uracil. Histones (B) are proteins that DNA wraps around in chromosomes, not structural units of nucleic acids. Nucleosides (A) are composed of a nitrogenous base and a sugar molecule but lack the phosphate group present in nucleotides. N-bases (D) is not a recognized term in biology and does not accurately describe the structural units of nucleic acids.

Correct Answer: A

Rationale: The correct answer is A: ocular lens. The ocular lens is the lens closest to the eye in a compound light microscope. This lens is responsible for magnifying the image produced by the objective lens. It typically has a magnification power of 10x. The objective lens, choice B, is positioned closer to the specimen and provides the primary magnification of the image. The condenser, choice C, is located beneath the stage and helps focus light onto the specimen. The stage, choice D, is where the specimen is placed for observation. Therefore, the ocular lens is the correct choice as it is the lens closest to the eye and responsible for further magnifying the image.

Correct Answer: B

Rationale: The correct answer is B: cytopathic effect. When viruses infect cell cultures, they induce degenerative changes known as cytopathic effects, which include cell shrinkage, rounding, detachment, and cell death. This term specifically refers to the damage caused by viral replication within the cells. A: Cytolytic effect refers to the destruction of cells by viruses, which is a more specific type of cytopathic effect involving cell lysis. C: Cytotoxic effect typically refers to the toxic effects of substances on cells, not specifically related to viral propagation. D: The statement that all three answers are correct is incorrect as only option B accurately describes the degenerative changes resulting from viral propagation in infected cell cultures.