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
An elderly man presents with a history of jaundice and pain abdomen. The investigation of choice is: 
Select and explain. 
A. Ultrasound, B. 4-quadrant aspiration, C. CT scan, D. X-ray abdomen
A: Ultrasound
Answer: A: Ultrasound 
Explanation: Ans. Ultrasound