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': 'Phenol coefficient indicates \nSelect and explain. \nA. Efficacy of a disinfectant, B. Dilution of a disinfectant, C. Quantity of a disinfectant, D. Purity of a disinfectant', 'role': 'user'}, {'content': "Answer: A \nExplanation: On exposure to air, the colorless crystals of phenol become pinkish and on longer exposure, the color deepens to dark blue. Pure phenol is not an effective disinfectant. It is used as an effective disinfectant. Ref: 25th edition, Park's Textbook of Preventive and Social Medicine, Page no. 140", 'role': 'assistant'}]