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: Anaesthesia
Which of the following is the main component of soda lime used in a closed circuit? 
Select and explain. 
A. Sodium hydroxide, B. Barium hydroxide, C. Calcium hydroxide, D. Potassium hydroxide
C: Calcium hydroxide
Answer: C: Calcium hydroxide 
Explanation: Soda lime used in a closed circuit is a mixture of 5% sodium hydroxide and 95% calcium hydroxide. In a closed circuit, expired gas is passed through soda lime which completely absorbs carbon dioxide and the expired gas is then passed into a reservoir bag which is then reused. Absorbant should be changed when 50-70% has changed colour.