This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Which of the following drugs is used in severe hypertensive emergencies, is very short acting and must be given by i.v. infusion ? 
Select and explain. 
A. Diazoxide, B. Hydralazine, C. Labetalol, D. Nitroprusside
D: Nitroprusside
Answer: D: Nitroprusside 
Explanation: None