A 40-year-old woman has undergone mitral valve surgery for mitral regurgitation. She has been given vasopressin by infusion for systemic hypotension. Which of the following is the CORRECT mechanism of action by which vasopressin improves systemic blood pressure?

A) Binds to V1a receptors: vascular smooth muscle
B) Binds to V1b receptors: pituitary gland
C) Binds to V2a receptors: renal collecting duct permeability
D) Binds to V2b receptors: vascular endothelium