Which of the following is the best example of homeostasis in the human body?
A. The secretions from oil glands in the skin decrease as a person ages.
B. Opposite muscles are used to bend and extend a person 's arm at the elbow joint.
C. The villi of the small intestine increase the absorption of nutrients from the small intestine into the blood.
D. More growth hormone is released from the pituitary gland when the level of growth hormone in the blood falls too low.

Please explain why your answer is correct.