Acceleration is zero when velocity is constant

Steps:

• Identify the velocity-time graph segment from 10s to 20s.
• Note that the velocity remains constant in this interval.
• Recall that acceleration is the slope of the velocity-time graph.
• Conclude the slope is zero, so acceleration is zero.

Why D is correct:

• Acceleration is the rate of change of velocity (a = Δv/Δt); with constant velocity, Δv = 0, so a = 0.

Why the others are wrong:

• A: Velocity is constant, so acceleration does not decrease.
• B: Velocity is constant, so acceleration does not increase.
• C: Acceleration is zero, not constant but non-zero.

Final answer: D