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: Medicine
Brain death means loss of-a) Cortical functionb) Brain stem functionc) Spinal reflexesd) Corneal reflex 
Select and explain. 
A. acd, B. bcd, C. abc, D. abd
D: abd
Answer: D: abd 
Explanation: None