$A$ box of mass $m$ is released from rest at position $1$ on the frictionless curved track shown. It travels a distance $d$ along the track in time $t$ to reach position $2$,falling a vertical distance $h$. Let $v$ be the instantaneous speed and $a$ be the instantaneous acceleration of the box at position $2$. Which of the following equations is valid for this situation?

• A
  $h = vt$
• B
  $h = (1/2)gt^2$
• C
  $d = (1/2)at^2$
• D
  $mgh = (1/2)mv^2$