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: Pharmacology
[{'content': 'Tetracycline acts on: \nSelect and explain. \nA. 30S ribosomes, B. 50S ribosomes, C. Cell membrane, D. Inhibiting DNA gyrase', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]