Which of the following describes a positive feedback loop? A. When blood sugar concentration is too low, the endocrine system secretes hormones that increase blood sugar concentration to a normal level.
B. When the body senses that the heart rate is very high, it secretes hormones that decrease the heart rate.
C. When blood sugar concentration is too high, the endocrine system secretes hormones that decrease blood sugar concentration to a normal level.
D. When someone is exercising intensely, the body secretes hormones that increase oxygen supply for cellular respiration.