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
How many fully frozen ice packs should a vaccine carrier contain? 
Select and explain. 
A. 2, B. 4, C. 6, D. 8
B: 4
Answer: B: 4 
Explanation: - Vaccine carrier contain 4 fully frozen ice-packs - Day carrier contain 2 fully frozen ice-packs