A voltmeter was used to measure the potential difference across the cell membrane of a giant squid axon. 

Which of the following best explains the voltmeter reading?

Select one:

a. The sodium ions have rushed into the axon, and the sodium channels are closing.

b. The sodium potassium exchange pumps are working to take out the sodium ions and bring in the potassium ions.

c. The potassium ions have rushed out of the axon, and the potassium channels are slow to shut down.

d. The threshold potential has not been reached.

My answer is C but im not sure if it is correct or not.