This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
Vaccine which requires most stringent condition for storage:(1991) 
Select and explain. 
A. DPT, B. OPV, C. BCG, D. TT
B: OPV
Answer: B: OPV 
Explanation: Ans: bRef: Park, 17th ed, p. 93 & 18th ed, p. 99