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
The investigation of choice for acute cholecystitis is: 
Select and explain. 
A. USG, B. HIDA-Scan, C. CT-Scan, D. OCG
A: USG
Answer: A: USG 
Explanation: A i.e. USG