Why will your vehicle's speed naturally increase on downgrades?

Answer: D — Gravity

On a downgrade, the force of gravity pulls your vehicle downhill, which causes it to speed up without needing to press on the accelerator. Air resistance, engine efficiency, and tire pressure do not have a direct impact on your vehicle's speed on downgrades. While air resistance may have some minimal effect, it is not significant enough to cause a noticeable increase in speed. Engine efficiency and tire pressure can affect overall vehicle performance, but they do not directly cause an increase in speed on downgrades. Therefore, the main factor contributing to a natural increase in speed on downgrades is gravity.