Which of the following is an example of a positive feedback?
a. a low level of oxygen in the blood triggers production of red blood cells, which transport oxygen. b. an increase in blood sugar level stimulates the pancreas to secrete insulin, which lowers blood sugar level. c. a tear in the lining of a blood vessel triggers the accumulation of blood platelets, which cause blood to clot.

C. A tear in the lining of a blood vessel triggers the accumulation of blood platelets, which cause blood to clot.

Explanation:

Positive feedbakc occurs when something happens that triggers B, this B then causes the first thing that originally happened to happen again, and it is a vicious circle that harms the system into unestability. For example the birth circle, when brain releases ocytocin to create contractions and then contractions make the body realese prostaglandis, and this causes the body to realease more oxytocin, which starts the cycle again.