13. which statement correctly describes the human heart? a. it has three chambers, and pumps blood directly into veins. b. it has two chambers, and it is composed of cardiac muscle. c. it has three chambers, and it is composed of cardiac muscle. d. it has four chambers, and pumps blood directly into arteries. 14. an individual who has had chicken pox rarely gets this disease again. this situation is an example of a. biological control b. negative feedback c, immunity d. metamorphosis 15. the nervous system is most directly involved in a. production of blood cells b. stimulation of rapid growth movement c. elimination of body wastes d. initiation of muscle