Several litres of acid were drawn off a 54-litre vessel full of acid and an equal amount of water was added. Again the same volume of the mixture was drawn off and replaced by water. As a result, the vessel contained 24 litres of pure acid. How much of the acid was drawn off initially?

• A. 12 litres
• B. 16 litres
• C. 18 litres
• D. 24 litres