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
Usually healthy human volunteers are taken in: 
Select and explain. 
A. Phase I of clinical trial, B. Phase II of clinical trial, C. Phase III of clinical trial, D. Phase IV of clinical trial
A: Phase I of clinical trial
Answer: A: Phase I of clinical trial 
Explanation: None