Which of these is an example of negative feedback?

a. As a blood clot begins to form, the process of its formation gets faster and faster.
b. the "thermostat" in the hypothalamus of mammals shuts off shivering once normal body temperature is reached
c. The digestive enzyme pepsinogen is converted to pepsin by the action of hydrochloric acid; pepsin itself can then convert pepsinogen into pepsin.
d. Once labor during childbirth begins, uterine contractions increase in frequency and intensity to push the baby out