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
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? 
Select and explain. 
A. Hemothorax, B. Pneumothorax, C. Cardiac tamponade, D. Pulmonary embolism
B: Pneumothorax
Answer: B: Pneumothorax 
Explanation: None