13. Which of the following statements describes an action potential?
A. An action potential moves in one direction on a neuron and is very fast, moving down a neuron in a fraction of a second.
B. An action potential moves in both directions on a neuron and is very fast, moving up or down a neuron in a fraction of a second.
C. An action potential moves in one direction on a neuron and its speed is variable depending on the strength of the signal.
D. An action potential moves in both directions on a neuron and its speed is variable depending on the strength of the signal.