The contraction and expansion of the walls of food pipe is called movement.

Correct Answer: A

Rationale: Peristalsis is a type of wave-like movement occurring in the alimentary canal which involves contraction and relaxation of the muscles. It begins in the esophagus and helps to move the food along the alimentary canal for the process of digestion.

Correct Answer: D

Rationale: In the mouth, food undergoes mechanical digestion via chewing, chemical digestion through salivary amylase, and dissolving of nutrients.

Correct Answer: A

Rationale: In the lock and key model, the enzyme (amylase) is the key that specifically binds to the substrate (starch), acting as the lock.

Correct Answer: A

Rationale: Bacterial metabolism of food releases acids, lowering the pH in the mouth.

Correct Answer: D

Rationale: Sebum is an oily substance produced by sebaceous glands in the skin, not by gastric glands, which secrete HCl, intrinsic factor, and pepsin.