$A$ railway track is banked for a speed $v$ by elevating the outer rail by a height $h$ above the inner rail. If the distance between the two rails is $d$,then the radius of curvature of the track is ($g$ = gravitational acceleration).

• A
  $\frac{v^2 d}{gh}$
• B
  $\frac{2 v^2}{gdh}$
• C
  $\frac{gd}{2 v^2 h}$
• D
  $\frac{v^2}{2 gh d}$