A ball of mass $m$ drops straight down and strikes a hard surface. The speed of the ball as it reaches the surface is $v_1$. It then bounces straight up with speed $v_2$. What is the change in momentum of the ball?
- A$mv_1$
- B$mv_2$
- C$m(v_1 + v_2)$
- D$m(v_2 - v_1)$