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': 'Phase 4 clinical trial is carried out: \nSelect and explain. \nA. Before the marketing approval of a drug, B. After a drug is marketed, C. For drugs used in rare diseases, D. For drugs used in pediatric patients', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]