Salivary secretion:

Correct Answer: A

Rationale: The correct answer is A because salivary secretion contains enzymes like amylase that start the digestion of carbohydrates in the mouth. This enzyme breaks down complex carbohydrates into simpler sugars. Choices B, C, and D are incorrect because salivary secretion is not directly controlled by secretin hormone, does not start the digestion of proteins, and is not highly acidic. Salivary secretion primarily aids in the initial breakdown of carbohydrates, preparing them for further digestion in the stomach and intestines.

Correct Answer: C

Rationale: The correct answer is C: Mucosa and Lumen. The mucosa surface cell secretes mucus that forms a physical barrier between the mucosa (inner lining of a cavity or organ) and the lumen (inner space of a tubular structure). This barrier protects the underlying tissues from harmful substances in the lumen. Choice A (Mucosa and Epithelium) is incorrect because the mucosa is part of the epithelium layer, so it would not make sense for the physical barrier to be between them. Choice B (Lymph Vessel and Lamina Propria) is incorrect because these structures are not directly related to the secretion of mucus for protection. Choice D (Lumen and Epithelium) is incorrect because the physical barrier created by the mucus is specifically between the mucosa cells and the lumen, not the epithelium itself.

Correct Answer: C

Rationale: The majority of nutrient absorption occurs in the small intestine. Here's the rationale: 1. The small intestine is where most digestion and absorption of nutrients occur due to its large surface area. 2. Villi and microvilli in the small intestine increase the absorption surface for nutrients like carbohydrates, proteins, and fats. 3. Specific enzymes and bile are secreted in the small intestine to aid in the breakdown and absorption of nutrients. 4. The large intestine primarily absorbs water and electrolytes, not nutrients. In summary, the mouth is mainly for mechanical digestion, the stomach primarily digests food, and the large intestine absorbs water, making the small intestine the correct choice for nutrient absorption.

Correct Answer: B

Rationale: The correct answer is B: Liver. The liver produces bile, which is a greenish-yellow fluid that helps in the digestion and absorption of fats. It is stored in the gallbladder and released into the small intestine when needed. The stomach (A) produces gastric juices, not bile. The pancreas (C) produces digestive enzymes, but not bile. The small intestine (D) is where bile is released to aid in the digestion of fats, not where it is produced or stored.

Correct Answer: B

Rationale: The correct answer is B: liver. The liver is not part of the gastrointestinal tract. The GI tract consists of the esophagus, stomach, and small intestine, where digestion and absorption of nutrients occur. The liver is an accessory organ that aids in digestion by producing bile but is not part of the tract itself. The liver is involved in detoxification, storage of nutrients, and production of important proteins, but it does not play a direct role in the digestion process. Therefore, the liver is the correct answer.