What type of driving condition requires the use of low gears?

The correct choice emphasizes the necessity of using low gears specifically in situations involving steep downgrades and heavy loads. When driving a tractor-trailer, low gears are essential for maintaining control and managing speed effectively when descending steep slopes. This is crucial because low gears provide increased engine braking, which helps slow down the vehicle without excessive reliance on the brake system, ultimately preventing brake overheating and failure.

In addition, when hauling heavy loads, low gears allow for better torque and power application, which is vital for the vehicle's ability to move and stop safely under such conditions. The engine's increased resistance in low gear helps in navigating the weight and preventing dangerous situations that could arise from losing control.

In contrast, the other situations presented do not necessitate the use of low gears. Driving in heavy traffic often requires frequent gear changes but does not specifically demand low gears for safety. Similarly, highway driving at high speeds benefits from higher gears to improve fuel efficiency, and driving on flat, dry roads typically allows for the use of higher gears for optimal performance.