What is a primary reason for using air brakes in large vehicles?

Answer: C — Ability to control speed on steep grades

The use of air brakes in large vehicles is primarily due to their ability to effectively control speed on steep grades. When navigating downhill slopes, maintaining control over a heavy vehicle can be challenging due to the sheer weight and momentum. Air brakes provide substantial stopping power, allowing the driver to apply consistent and controlled braking force, which is crucial for safety. This system uses compressed air to operate the brakes, enabling greater braking force without the risks associated with other braking systems, such as brake fade. While lower maintenance costs, greater power and effectiveness, and improved fuel efficiency are considerations for various braking systems, the critical function of air brakes in managing vehicle speed on steep grades is indispensable, especially in the context of large trucks and buses. This capacity enhances the overall safety and reliability of these vehicles when navigating challenging terrain.