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
All the following drugs are used in hepatic encephalopathy, except -a) Mannitolb) Metronidazolec) Lactulosed) Phenobarbitone 
Select and explain. 
A. b, B. c, C. ad, D. ab
C: ad
Answer: C: ad 
Explanation: None