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': 'Cholera transmission by -a) Food transmitsb) Vaccination gives 90% efficiencyc) Healthy carrierd) Chlorination is not effective \nSelect and explain. \nA. a, B. bc, C. ac, D. b', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]