Final answer:
The correct answer is D, which states that on a downgrade, a vehicle is working with gravity, leading to an increase in speed. This is due to gravity causing positive acceleration along the downward path.
Step-by-step explanation:
The correct answer is option D. On a downgrade, your vehicle is working with gravity, resulting in a(n) increase in speed. When a car is moving down a slope, gravity pulls it along the downward path, causing it to accelerate unless other forces, such as friction or air resistance, are sufficient to counteract this acceleration.
Just as a car moving toward the right with a negative acceleration is decelerating, a car moving downhill with gravity is experiencing positive acceleration, assuming that the direction of gravity is considered positive in this context.
Understanding that acceleration is defined as a change in velocity, if a vehicle's velocity is becoming less negative (as in the case of Car E, which goes from -25 m/s to -5 m/s), it is considered to be accelerating.
Conversely, when a vehicle's velocity is becoming more negative (like Car D speeding up in the negative direction), it still indicates a form of acceleration, though in this case, it's negative acceleration.
Therefore, a vehicle on a downgrade typically accelerates due to the force of gravity pulling it downward, increasing its speed unless opposed by sufficient frictional forces.