Constant speed from distance-time graph implies constant kinetic energy

Steps:

• Distance-time graph shows straight line, indicating constant speed greater than zero.
• Constant speed means velocity magnitude is unchanging and positive.
• Kinetic energy depends on speed squared, so remains constant and positive.
• Uphill travel increases height, but speed constancy keeps kinetic energy steady.

Why D is correct:

• Kinetic energy formula $KE = \frac{1}{2}mv^2$ shows it depends only on mass and speed; constant speed $v > 0$ keeps constant and greater than zero.

Why the others are wrong:

• A. Acceleration: Constant speed means zero acceleration, not greater than zero.
• B. Displacement: Increases steadily as the car moves up the hill.
• C. Gravitational potential energy: Increases with height gained on the uphill path.

Final answer: D