This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cystic fibrosis & Sleep apnea, Subject name: Medicine
Severe obstructive sleep apnea is defined as an apnea-hypopnea index of greater than? 
Select and explain. 
A. 10 events/ hour, B. 15 events/ hour, C. 20 events/ hour, D. 30 events/ hour
D: 30 events/ hour
Answer: D: 30 events/ hour 
Explanation: Obstructive sleep apnea/ hypopnea syndrome quantification and severity scale is graded according to number of apnea/ hypopnea index events per hour Mild OSAHS 5 -14 events / hour Moderate OSAHS 15-29 events / hour Severe OSAHS >30 events / hour