The time interval between action potentials is called the

Hello,
  I am about half way through this lab and REALLY struggling.  I can't find any other posts about it and need help.  Its from Mastering A&P online.  I tried to copy some questions on here, but my computer wouldn't let me.  It is exercise 3 activity 1-9.  What I have found on her for those exercises, have been th wrong qestions for me.  Here is an example for a question for : Exercise 3 > Activity 6: Activity and Post-lab Quiz for The Action Potential: Coding for Stimulus Intensity
1. The time interval between action potentials is called the _____
 potential frequency
threshold frequency
threshold interval
interspike interval
Please help!!