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 has poorest oral bioavailability ? 
Select and explain. 
A. Oseltamivir, B. Zanamivir, C. Rimantidine, D. Amantadine
B: Zanamivir
Answer: B: Zanamivir 
Explanation: None