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
A 45 year old female patient returns to your clinic with complaint of food lodgement in interproximal area of 47. On intraoral examination, 47 is restored with mesio-disto-occlusal amalgam restoration. Which type of matrix is used for this preparation? 
Select and explain. 
A. Automatrix, B. Compound supported matrix, C. Universal matrix, D. Ivory matrix
A: Automatrix
Answer: A: Automatrix 
Explanation: When little tooth structure remains and deep gingival margins are present, the Tofflemire matrix may not function successfully, and the Automatrix system is beneficial.