Final answer:
The primary factor to consider when selecting the gear for traveling up steep grades is the steepness of the grade.
Step-by-step explanation:
The primary factor that should help you determine what gear to select while traveling up steep grades is A. The steepness of the grade.
The steepness of the grade affects the amount of force needed to overcome gravity and move the vehicle uphill. steeper grades require lower gears to provide more torque, while shallower grades allow for higher gears and faster speeds.
The weight of the load on the vehicle and the length of the grade also play a role in gear selection, but the steepness of the grade is the main factor.