This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: CNS, Subject name: Medicine
A 27-year-old male presents to the emergency with severe pain behind his right eye for the last 30 minutes. He states that his right eye has also been 'watering excessively' for the last two hours. The man has had similar episodes over the last few weeks, but this time the pain is significantly worse which has prompted him to present to the ER. The vitals are normal and ophthalmic examination demonstrates conjunctival injection and miosis of the right pupil. A CT scan of the brain and ESR are unremarkable. Which is the initial treatment of this patient? 
Select and explain. 
A. Sumatriptan, B. Rizatriptan, C. Steroids, D. High flow oxygen
D: High flow oxygen
Answer: D: High flow oxygen 
Explanation: Retro orbital pain: Cluster headache (Pain behind the eye) | 1st line of treatment: High flow oxygen at 12-15L/min To prevent future episodes of cluster headache: Verapamil Sumatriptan/Rizatriptan: 1st line treatment for Migraine headache. Steroids: Prevention of fuher attacks.