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
What are 'G-proteins'? 
Select and explain. 
A. Membrane receptors, B. Membrane regulators, C. Second messengers, D. Membrane channels
B: Membrane regulators
Answer: B: Membrane regulators 
Explanation: None