Correct Answer: C

Rationale: The correct answer is C, the glomerulus. The glomerulus is a cluster of capillaries in the kidney responsible for filtering blood as it enters the nephron. It plays a crucial role in the initial stages of urine formation by filtering waste products and excess substances from the blood. Choice A, the Bowman's capsule, is incorrect as it surrounds the glomerulus and captures the filtrate. Choice B, the Loop of Henle, is incorrect as it is part of the nephron responsible for water reabsorption. Choice D, the nephron, is incorrect as it comprises the functional unit of the kidney and includes the glomerulus, Bowman's capsule, and other structures.

Correct Answer: B

Rationale: The correct answer is B. In this experiment, the independent variable is the amount of poison because it is what is being manipulated by the researcher. The dependent variable is how quickly the rat dies, as it is the outcome that is being measured based on the different amounts of poison administered. Choice A is incorrect because the independent variable should be what is being manipulated or changed, which is the amount of poison in this case. Choice C is incorrect because whether the rat eats the poison is not being varied or controlled by the researcher. Choice D is incorrect because the cage the rat is kept in is not relevant to the relationship being studied between the amount of poison and the rat's survival time.

Correct Answer: A

Rationale: The sensory threshold refers to the smallest amount of stimulus required for an individual to perceive or feel a sensation. It is the minimum level of stimulus intensity that is detectable by an individual. Choice B is incorrect because the sensory threshold is not specifically related to feeling pain but rather to perceiving any sensation. Choice C is incorrect as the sensory threshold is about perception and not necessarily physical reaction. Choice D is incorrect as it describes the source of the stimulus, not the threshold for perception.

Correct Answer: A

Rationale: A control is a standard used to compare the outcomes of an experiment and is crucial for determining the effect of the variables being tested. It helps isolate and measure the impact of the independent variable by providing a baseline for comparison. While variables may change, constants remain consistent, and collected data is the information obtained from the experiment. A control specifically serves as the standard for comparison in an experiment, making it the correct choice. Variables, although important, are what are being altered and tested in an experiment, constants are factors that are kept consistent throughout the experiment, and collected data is the information gathered from the experiment, none of which specifically serve as the standard for comparison.

Correct Answer: B

Rationale: The correct answer is B: Secondary. The secondary structure of a protein is defined by the folds and coils of the protein's polypeptide backbone. This level of structure is characterized by the formation of alpha helices and beta sheets, which are stabilized by hydrogen bonds between amino acids along the polypeptide chain. Choice A, Primary, refers to the linear sequence of amino acids in the protein. Choice C, Tertiary, involves the 3D folding of the entire polypeptide chain. Choice D, Quaternary, pertains to the arrangement of multiple polypeptide subunits in a protein complex.