This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anaesthetic equipments, Subject name: Anaesthesia
Inner diameter of adult male endotracheal tube is 
Select and explain. 
A. 2 mm, B. 4 mm, C. 6 mm, D. 9 mm
D: 9 mm
Answer: D: 9 mm 
Explanation: Internal diameter of the endotracheal tube used usually for an adult male ranges from 8.0 to 9.5 ID for an adult female ranges from 6.5 to 7.5