The principal cause of male infertility is:

Correct Answer: B

Rationale: A varicocele (B) is the leading cause of male infertility, impairing sperm production. Others (A, C, D) are less common.

Correct Answer: B

Rationale: False (B): Mammary glands are modified sweat glands, not sebaceous. True (A) is incorrect.

Correct Answer: B

Rationale: False (B): Secretory phase builds endometrium; menstrual phase discharges it. True (A) is incorrect.

Correct Answer: B

Rationale: False: Epiglottis elevates (not depresses) to cover glottis (B). Slack cords lower pitch (A), whispering avoids vibration (C), testosterone enlarges larynx (D).

Correct Answer: B

Rationale: Elevated CO₂ causes respiratory acidosis (B) from H⁺ increase. Breathing increases (not decreases, A, D), pressure receptors (C) are unrelated.