Biliverdin and bilirubin are pigments that result from the breakdown of

A. They are secreted in an inactive form: major proteases of the GI tract (pepsinogen, trypsinogen, chymotrypsinogen, etc.) are secreted as zymogens (inactive precursors) to prevent autodigestion.
B. They are activated by HCl: pepsinogen is activated by HCl, but pancreatic proteases are activated by trypsin, not HCl, so this is not true for all protein-digesting enzymes.
C. They are secreted by the pancreas: many proteases (e.g., pepsin) are secreted by the stomach, so not all are pancreatic.
D. Their release is stimulated by enterokinase: enterokinase (enteropeptidase) activates trypsinogen to trypsin in the small intestine but does not stimulate the release of all proteases; it is an activator, not a universal release stimulus.

A. Mononuclear phagocytic system: that system (macrophages/monocytes) performs cellular phagocytosis, not the physical barriers listed.
B. Chemical barriers: some secretions (tears, saliva) contain chemical antimicrobials, but the items listed (mucus, hair) are primarily physical/mechanical barriers.
C. Adaptive defenses: adaptive defenses are specific immune responses (antibodies, T cells), not the nonspecific physical barriers listed.
D. Mechanical barriers: mucus, hair, and the physical flow of tears/saliva act as mechanical/physical barriers that trap or remove pathogens before they invade.