What primary factor should determine the gear selection when traveling up steep grades?

Answer: D — Both the steepness of the grade and weight of the load

When selecting the appropriate gear while traveling up steep grades, both the steepness of the grade and the weight of the load play crucial roles. The steepness of the grade affects how much power is required to maintain speed and climb effectively. A steeper grade requires more torque, which can demand lower gears to provide that necessary power. Additionally, the weight of the load significantly influences gear selection. Heavier loads require more power to move, which again leads to a need for lower gears to ensure that the vehicle has enough force to ascend the hill without losing momentum or stalling. By considering both the steepness of the grade and the weight of the load, drivers can make informed decisions about which gear to select, ensuring safe and efficient climbing. This combined assessment is essential for effective vehicle control and for preventing strain on the engine and transmission during challenging driving conditions.