Which system includes the brain, nerves, and spinal cord?

The correct answer is A, the Nervous System. The Nervous System is a complex network that includes the brain, spinal cord, and a vast array of nerves that extend throughout the body. This system is responsible for coordinating and controlling the body's responses to internal and external stimuli.

The brain acts as the control center, processing information and making decisions, while the spinal cord transmits signals between the brain and the rest of the body. Nerves carry these signals to and from various parts of the body, allowing for communication and coordination of voluntary actions as well as involuntary processes. This intricate system ensures that the body can respond effectively to changes in the environment, maintain homeostasis, and manage both conscious and reflexive actions.

Other systems mentioned do not include these components. The Excretory System focuses on the removal of waste from the body, the Respiratory System is involved in gas exchange, and the Circulatory System is responsible for the transportation of blood and nutrients. None of these systems encompass the brain, spinal cord, or nerves, which are key elements of the Nervous System.