This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cold Chain in India, Subject name: Social & Preventive Medicine
All are true about fully frozen Ice pack except:- 
Select and explain. 
A. Smallest component of cold chain, B. Two vaccines are kept in two holes inside the ice pack, C. Fill water up to horizontal mark with plain tap water, D. Can store vaccines for 24hrs
D: Can store vaccines for 24hrs
Answer: D: Can store vaccines for 24hrs 
Explanation: Fully Frozen Ice Pack; Fill water up to horizontal mark with plain tap water. Smallest Component of cold chain. If there is any leakage such ice-packs should be discarded. 2 vaccines are kept in two holes inside the ice pack. Can maintain vaccine temeprature for 2-4 hours.