On the graph, the first action you see is a straight line going up. The key word you look for in the word explanations are "increase" and "constant rate". Then, the 2nd action you see is a straight line not going up or down, just staying on the same level. This could be called "remaining the same" or stopping/resting. Then, you see a straight line going down. You look for "decrease" at a "constant rate." D matches all of those.
