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: Microbiology
[{'content': 'In a patient of orbital cellulitis, micro organism on culture show greenish colonies and optochin sensitivity. The most likely organism is - \nSelect and explain. \nA. Strep. viridans, B. Staphylococcus, C. Pseudomonas, D. Pneumococcus', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]