Uniform acceleration produces a linear speed-time graph

Steps:

• Uniform acceleration means constant change in speed over time.
• On a speed-time graph, speed is the y-axis and time is the x-axis.
• Constant acceleration results in a straight line with positive slope for increasing speed.
• Horizontal track implies no vertical motion, so focus on horizontal acceleration.

Why A is correct:

• A straight line with positive slope matches the definition of uniform acceleration, where $v = u + at$ yields linear speed versus time.

Why the others are wrong:

• B: Curved line indicates non-uniform (changing) acceleration.
• C: Horizontal line shows constant speed (zero acceleration).
• D: Straight line with negative slope or curve represents deceleration or varying motion.

Final answer: A