While Dr Jackson Is Recording the Activity of a Squid Axon

Multiple Choice

 While Dr. Jackson is recording the activity of a squid axon, he finds that a larger depolarizing shock is necessary on occasion in order to produce a new action potential. What is the most likely explanation for Dr. Jackson's observations?

A) The squid axon is such an artificial laboratory preparation, it cannot be expected to respond like an axon in a living animal.
B) Dr. Jackson's second shock probably occurs during a relative refractory period.
C) Dr. Jackson's second shock probably occurs during an absolute refractory period.
D) The squid axon in the laboratory does not have functioning sodium-potassium pumps.