The flow rate of air is the amount of air that passes through a system _____________.

The flow rate of air refers specifically to the volume or mass of air that moves through a pneumatic system within a defined time frame, which is why "in a certain amount of time" is the correct completion of the statement. Flow rate is typically measured in units such as liters per minute (L/min) or cubic feet per minute (CFM), indicating not just the quantity of air but also the temporal aspect of its movement. Understanding this concept is crucial for evaluating system performance, selecting appropriate components, and ensuring that a pneumatic system operates efficiently to meet its intended application.

The other choices do not address the fundamental nature of flow rate. While temperature can affect air density and thus its flow behavior, it does not define the flow rate itself. The stipulation about air not being returned to atmosphere is unrelated to the definition of flow rate, as flow rate can apply regardless of whether the air is contained within a closed system or being vented. Lastly, the status of the compressor being started or not is irrelevant to the definition of flow rate, as flow can be measured before a system is fully operational.