Correct Answer: A

Rationale: The correct answer is A: Villi. Villi are small fingerlike projections in the intestine that increase the surface area for nutrient absorption. Goblets (choice B) are mucus-secreting cells in the intestines. Lacteals (choice C) are lymphatic vessels in the small intestine that absorb dietary fats. Polyps (choice D) are abnormal tissue growths.

Correct Answer: D

Rationale: The correct answer is D, 'Mastication.' Mastication is the mechanical process of chewing that breaks down food into smaller particles. Peristalsis (choice A) is the involuntary constriction and relaxation of muscles in the gastrointestinal tract that moves food along. Segmentation (choice B) is the mixing movement of the digestive system that helps to break down food and mix it with digestive juices. Metabolism (choice C) refers to the chemical processes that occur within a living organism to maintain life.

Correct Answer: D

Rationale: The correct answer is D: Hydrochloric acid. Hydrochloric acid in the stomach plays a crucial role in converting pepsinogen into its active form, pepsin, which is necessary for digesting proteins. Bile (Choice A) is involved in the emulsification of fats, not in the activation of pepsinogen. Gastrin (Choice B) is a hormone that stimulates the secretion of gastric acid but does not directly activate pepsinogen. Secretin (Choice C) is a hormone that regulates the pH of the duodenum but is not responsible for the activation of pepsinogen.

Correct Answer: B

Rationale: In a low-carbohydrate diet, fats become the primary source of calories as carbohydrate intake is restricted. Proteins can contribute to caloric intake but are not the primary source in a low-carbohydrate diet. Carbohydrates are intentionally limited in this diet, so they do not supply most of the calories. Vitamins are essential nutrients but do not provide calories.

Correct Answer: B

Rationale: The correct answer is B: nonessential amino acids. Nonessential amino acids can be synthesized by the body, so they do not need to be obtained through the diet. Essential amino acids, choice A, must be obtained from the diet because the body cannot produce them. Choices C and D, complete and incomplete amino acids respectively, do not accurately describe the classification of amino acids based on synthesis by the body.