On a velocity-time graph, why is the line straight? (Physics)

Question

I know that a straight line means constant acceleration on the vt graph. My assignment question is asking to "briefly explain" but I dont know how to wxplain it any further then " a straight line means constant acceleration" 

Answer 1

A st. line graph has constant slope, in this case m = change in velocity/change in time. For equal time intervals (of change), the change in velocity is the same constant. That's the definition of acceleration (Δv / Δt).