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: Pharmacology
True about naltrexone is all except: 
Select and explain. 
A. Acts on opioid receptors, B. Is used in treatment of alcohol dependence, C. Is used to reduce craving in dependence, D. Is an opioid agonist
D: Is an opioid agonist
Answer: D: Is an opioid agonist 
Explanation: None