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: Medicine
[{'content': 'Pulsus alternans is: \nSelect and explain. \nA. An innocuous feature, B. Denotes severe cardiac decompensation, C. Sign of Digoxin toxicity, D. Sign of mitral incompetence', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]