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: Surgery
[{'content': 'Pseudomembranous colitis is caused by \nSelect and explain. \nA. Cl. perfringens, B. Cl. difficile, C. Cl. oedematicus, D. Si. septicum', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]