You can expect a control system within a building to turn mechanical devices on and off and to adjust A) settings when devices fail. B) the primary source of power. C) the output of mechanical devices. D) the level of noise.

To determine the correct answer, let's analyze the options in the context of a control system within a building: A) **Settings when devices fail**: While a control system may monitor devices and potentially alert when they fail, it typically does not adjust settings automatically in response to failures. B) **The primary source of power**: Control systems do not usually control the primary power source itself; they manage the devices that use power. C) **The output of mechanical devices**: This is a primary function of control systems. They are designed to adjust the operation of mechanical devices, such as HVAC systems, to maintain desired conditions (like temperature or humidity). D) **The level of noise**: While control systems can indirectly affect noise levels by managing devices, they do not specifically control noise levels as a primary function. Based on this analysis, the best answer is: **C) the output of mechanical devices.**