Which of These Is an Example of a Positive-Feedback Mechanism

Which of these is an example of a positive-feedback mechanism?

A) An increase in blood pressure activates mechanisms that decrease blood pressure.
B) Increased amounts of a hormone in the blood cause a decrease in the secretion of that hormone.
C) Increased carbon dioxide in the blood increases breathing rate, which decreases carbon dioxide in the blood.
D) Increased amounts of fluid in the blood result in increased quantities of urine, which decreases fluid content of the blood.
E) Increased stretch of the uterus causes it to contract, which further increases stretch.