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
Insulin having longest duration of action is: 
Select and explain. 
A. Isophane insulin, B. Protainine zinc insulin, C. Insulin zinc suspension, D. Plain insulin
B: Protainine zinc insulin
Answer: B: Protainine zinc insulin 
Explanation: None