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: Radiology
[{'content': 'A patient of RTA with injury over chest and limbs has low SP02. M-mode ultrasound of right upper chest shows stratosphere sign. What is the diagnosis? \nSelect and explain. \nA. Hemothorax, B. Pneumothorax, C. Cardiac tamponade, D. Pulmonary embolism', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]