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
30 years old lady presented with acute pain abdomen, constipation and vomiting suspecting acute intestinal obstruction. The investigation of choice for the patient is - 
Select and explain. 
A. Ba enema, B. X-ray abdomen erect posture, C. USG, D. CT scan
B: X-ray abdomen erect posture
Answer: B: X-ray abdomen erect posture 
Explanation: Ans. is 'b' i.e. X-ray abdomen erect posture