Why is the inside of a nerve cell negative?

Which of the following is an example of this type of co-evolution

Plants transfer carbon in the carbon cycle a. through assimilation of carbon from the soil. b. when carbon transpires from their stomatae. c. when they are eaten by animals. d. through fixation of carbon in the soil. plants transfer carbon in the carbon cycle a. through assimilation of carbon from the soil. b. when carbon transpires from their stomatae. c. when they are eaten by animals. d. through fixation of carbon in the soil.

In figure 1-2 what is the distance from point b to point c?