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: Social & Preventive Medicine
In Indoor air pollution, carbon monoxide is produced by – 
Select and explain. 
A. Combustion equipment, B. Stove, C. Gas heaters, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None