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
Trials stage for experimentation of adverse drug reaction in large scale population is ? 
Select and explain. 
A. I, B. II, C. III, D. IV
D: IV
Answer: D: IV 
Explanation: Ans. is 'd' i.e., IV o Long-term adverse effects on large number of patients -f Phase IV clinical trial.