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: Social & Preventive Medicine
[{'content': 'The efficiency of cold chain system for oral polio vaccine as monitored by Vaccine Vial Monitor (VVM) depends on \nSelect and explain. \nA. Change in colour of monitor, B. Temperature indicator of the system, C. Viral potency test, D. Change in the colour of vaccine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: WHO grading of VVM in OPV: (Marker of pofency)\nIs based on colour changes in VVM: ONLY INNER SQUARE CHANGES COLOUR, circle always remain blue.', 'role': 'assistant'}]