Which of the following processes is an example of positive feedback?

Which of the following processes is an example of positive feedback? a. High CO2 blood levels stimulate respiration which decreases blood CO2 levels b. High blood glucose levels stimulate insulin release, which makes muscle and liver cells take in glucose c. Increased nursing stimulates increased milk production in mammary glands d. Low blood oxygen levels stimulate erythropoietin production which increases red blood cell production by bone marrow