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: Dental
The most common cause of fracture at the isthmus of a class II dental amalgam restoration: 
Select and explain. 
A. Delayed expansion, B. Inadequate depth at the isthmus area, C. Inadequate width at the isthmus area, D. Moisture contamination of the amalgam
B: Inadequate depth at the isthmus area
Answer: B: Inadequate depth at the isthmus area 
Explanation: None