Which of the following best illustrates the concept of positive feedback?

a. The secretion of acid by cells in the stomach lining is inhibited when the pH of the stomach contents increases.

b. A rise in blood pressure stimulates the elimination of water in the urine, which reduces blood pressure.

c. Sodium channels are opened when a cation enters the cell. Opening of sodium channels results in sodium entering the cell.

d. An increase in the carbon dioxide concentration of the blood stimulates an increase in breathing, thus increasing the rate of carbon dioxide elimination from the body

e. An increase in plasma glucose stimulates secretion of insulin. Insulin facilitates transport of glucose from the plasma into cells, out of the plasma.