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
Which of the following does not act as second messenger ? 
Select and explain. 
A. Cyclic AMP, B. Inositol trisphosphate, C. Diacylglycerol, D. G proteins
D: G proteins
Answer: D: G proteins 
Explanation: None