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 A with injury over chest and limbs has low sp 02. M mode US of right upper pa of chest shows stratosphere sign. What is the diagnosis? 
Select and explain. 
A. Haemothotax, B. Pneumothorax, C. Cardiac tamponade, D. Pulmonary embolism
B: Pneumothorax
Answer: B: Pneumothorax 
Explanation: Ans. Pneumothorax