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 oxygen levels; the internal environment maintains all systems, including endocrine functions. C: Hormones are controlled by muscles; hormones are released when muscles contract and stopped when muscles relax. D: Hormones are controlled by the nervous system; the spinal cord sends messages about hormone levels to the brain.