Which of the following best summarizes how hormone levels are controlled? a

Hormones are controlled by a feedback loop; information about a system is sent back to the controller of the system to improve its function.

b

Hormones are controlled by the nervous system; the spinal cord sends messages about hormone levels to the brain.

c

Hormones are controlled by oxygen levels; the internal environment maintains all systems, including endocrine functions.

d

Hormones are controlled by muscles; hormones are released when muscles contract and stopped when muscles relax