Answer
(a) graph 3
(b) graph 2
Work Step by Step
The acceleration is the same for both rocks. It means that $\Delta v=v-v_0$ is the same for both rocks ($\Delta v=at$). Thus, both rocks have their speeds increased as they fall, but the difference in speeds remains the same. Their speed-versus-time plots must be two parallel straight lines.
Ball A initial speed: 0 -> plot 3
Ball B initial speed: $v_0$ -> plot 2
