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
A cancer patient who is on cisplatin therapy develops severe vomiting on the 1st day of treatment,  drug indicated to treat this is: 
Select and explain. 
A. Aprepitant., B. Ondansetron, C. Fosaprepitant., D. All of the above.
B: Ondansetron
Answer: B: Ondansetron 
Explanation: Cisplatin induced vomiting has two phases.

EarlyPhase: It occurs within first 24 hours. 5 HT3 antagonists like ondansetron are the agents of choice for this condition. 
Delayed Phase: Vomiting occurring after 24 hours is less responsive to ondansetron and other drugs. It is best controlled by substance P antagonist like aprepitant