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: Physiology
Ascending pain pathway is inhibited in dorsal midbrain by encephalin anda) 5-HT (serotonin)b) Noradrenalinec) Substance Pd) Glutamatee) Adenosine 
Select and explain. 
A. ac, B. a, C. ab, D. bc
C: ab
Answer: C: ab 
Explanation: Opioid interneurons inhibit the spinothalamic pain transmission after activation by serotonin and noradrenaline.