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 having the least oral bioavailability ? 
Select and explain. 
A. d–tubocurarine, B. Morphine, C. Ampicillin, D. Phenytoin
A: d–tubocurarine
Answer: A: d–tubocurarine 
Explanation: None