The Principle of Positive Feedback Is Illustrated by the Effect $\mathrm{PO}_{2}$

The principle of positive feedback is illustrated by the effect of

A) $\mathrm{PO}_{2}$ on breathing rate
B) glucose on insulin secretion
C) estrogen on follicle-stimulating hormone (FSH) secretion at midcycle
D) blood $\left[\mathrm{Ca}^{2+}\right]$ on parathyroid hormone (PTH) secretion
E) decreased blood pressure on sympathetic outflow to the heart and blood vessels