Which of These Is an Example of Chemical Gradient That

Which of these is an example of chemical gradient that exists in healthy living systems?

A) concentrations of Na⁺ inside the red blood cell and in the blood plasma
B) concentrations of K⁺ in blood and in cytoplasm of the muscle cell
C) concentrations of Ca²⁺ inside the sarcoplasmic reticulum and sarcoplasm of muscle cells between contractions
D) All of the choices are examples of chemical gradient.