The Diagram Shows an Action Potential for a Nerve Fiber $- 15$

The diagram shows an action potential for a nerve fiber. What is the threshold potential for this neuron?



A. $- 15$ millivolts
B. $- 50$ to $- 60 \quad$ Potential millivolts
C. $- 60$ to $- 70$ millivolts
D. $- 70$ to $- 80$ millivolts
E. $- 80$ millivolts