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: Physiology
Action of a - subunit of G protein is: 
Select and explain. 
A. Breakdown of GTP to GDP, B. Conversion of GDP to GTP, C. Internalization of receptors, D. Binding of agonist
A: Breakdown of GTP to GDP
Answer: A: Breakdown of GTP to GDP 
Explanation: A i.e. Breakdown of GTP to GDP