This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and Health, Subject name: Social & Preventive Medicine
[{'content': 'Which of the following OPV vials are usable? \nSelect and explain. \nA. Only 1, B. Only 1,2, C. Only 1,2, 3, D. Only 3, 4', 'role': 'user'}, {'content': 'Answer: B \nExplanation: (b) Only 1, 2Rules for VVM use in India* Rule 1: If the inner square is lighter than the outer circle, the vaccine may be used (Vials 1, 2)* Rule 2: If the inner square is the same colour as, or darker than, the outer circle, the vaccine must not be used (Vials 3,4).', 'role': 'assistant'}]