The presence of which of the following, in the alveoli ensure that particles brought in with inhaled air are removed?

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. External jugular vein: the external jugular drains superficial structures of the head and neck (including much of the face and scalp) and empties into the subclavian vein.
B. Superior vena cava: Incorrect (in the direct sense) -the superior vena cava is the large trunk that returns venous blood to the right atrium, but it does not directly drain the face/scalp; it receives blood ultimately via the brachiocephalic veins.
C. Subclavian vein: Partially related but not the primary answer -the subclavian receives blood from the external jugular, but it primarily drains the upper limb; it is a larger trunk rather than the direct superficial drain of face/scalp.
D. Cephalic vein: the cephalic vein is a superficial vein of the lateral upper limb (arm/forearm), not a drain of the head or face.