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 30 year old male presents to OPD with sensitivity to cold water. On intraoral examination, 36 is carious. When both a cavity varnish and a polycarboxylate cement base are being used in conjunction with a dental amalgam restoration, then the cavity varnish should be: 
Select and explain. 
A. Applied to the pulpal wall only, B. Kept well away from the enamel margins, C. Applied to the cavity preparation after placing the base, D. Applied to the cavity preparation before placing the base
C: Applied to the cavity preparation after placing the base
Answer: C: Applied to the cavity preparation after placing the base 
Explanation: When a cavity varnish or dentin bonding agent is indicated to seal a restoration, the base often governs the order of material application. If a zinc phosphate cement base is used, a sealing (Varnish) material should be applied to the cavity walls before placement of the base. For the more biocompatible cement base materials (e.g., calcium hydroxide, ZOE, polycarboxylate, and GIC), the base cement is placed, followed by the cavity varnish after the base has hardened.