A ball of mass $m$ drops straight down and strikes a hard surface. Its speed just before impact is $v_1$. It then bounces vertically upward with speed $v_2$. What is the change in momentum of the ball?
- A$m v_1$
- B$m v_2$
- C$m (v_1 + v_2)$
- D$m (v_2 - v_1)$