This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Storage of vaccines, Subject name: Pediatrics
The efficiency of cold chain system for oral polio vaccine as monitored by Vaccine Vial Monitor (VVM) depends on: 
Select and explain. 
A. Change in the colour of vaccine, B. Temperature indicator of the system, C. Viral potency test, D. Change in colour of monitor
D: Change in colour of monitor
Answer: D: Change in colour of monitor 
Explanation: Vaccine Vial Monitor (VVM) - Heat sensitive label that indicates cumulative heat exposure over time. - Whether the OPV l can be used or not depends upon the irreversible colour changes in vaccine l monitor. - If color of inside square lighter than outside circle - We can use the vaccine