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
In the setting of G-proteins which of the following is considered a third messenger : 
Select and explain. 
A. cAMP., B. Calcium., C. CO., D. NO.
B: Calcium.
Answer: B: Calcium. 
Explanation: Calcium is considered as a third messenger.