This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nerve Physiology, Subject name: Physiology
A travelling nerve impulse does not depolarize the area immediately behind it, because: 
Select and explain. 
A. It is hyperpolarized, B. It is refractory, C. It is not self-propagating, D. The condition is always orthodromic
B: It is refractory
Answer: B: It is refractory 
Explanation: Ans. is 'b' i.e. It is refractory Ganong writes -"Once initiated, a moving impulse does not depolarize the area behind it to the firing level, because this area is refractory"