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
[{'content': 'Reverse cold chain is used for \nSelect and explain. \nA. Carrying stool samples of polio patients from a PHC to the lab, B. Transpoing outdated vaccines from PHC to district hospital, C. Transpoing vaccine to lab, to check its potency, D. Transpoing vaccine from camps to sub - centre', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The cold chain is a system of storage and transpo of vaccines at low temperatures from the manufacturer to the actual vaccination site. The cold chain system is necessary because vaccine failure may occur due to failure to store and transpo under strict temperature control (refer pgno:109 park 23 rd edition)', 'role': 'assistant'}]