In dual air brake systems, what does the first or "primary" system refer to?

Answer: B — The rear brakes

In dual air brake systems, the first or "primary" system specifically refers to the rear brakes of the vehicle. This design is utilized to enhance safety and performance by ensuring that if one system fails, the other can still function, providing a backup. The primary system is responsible for controlling the rear braking mechanism, which is vital because most of the vehicle’s weight and stopping power is concentrated at the rear when it comes to braking. When a driver applies the brakes, the primary system engages first, allowing for more effective braking and reduced chances of skidding. This is particularly important in a large vehicle like a school bus, where maintaining control during braking is crucial for the safety of both the passengers and the driver.