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
Gold standard for reflux esophagitis is - 
Select and explain. 
A. Manometry, B. Barium swallow, C. 24 hours pH study, D. X-ray abdomen
C: 24 hours pH study
Answer: C: 24 hours pH study 
Explanation: None