A velocity vs. time graph for an object is shown.

A graph titled Velocity versus Time shows time in seconds on the x axis, numbered 0 to 5, Velocity in meters per second on the y axis, numbered 0 to 25. A line labeled W starts at the origin and ends at (1, 15). A line labeled X starts at (1, 15) and ends at (2, 20). A line labeled Y starts at (2, 20) and ends at (4, 20). A line labeled Z starts at (4, 20) and ends at (5, 0).
Which parts of the graph represents the object moving at a constant, positive acceleration?

A. W and X
B. X and Y
C. Y and Z
D. W and Z

(please help im so tired)